Electrical Problems

[Kev M]

Do we have an electrician in the house? I bought an ilford drum motor for processing film off eBay, it was sold in working condition but it sat on a shelf in work and I never got around to testing it until theother day when I couldn't get it to work. For some reason it has a two pin plug like a shaver, I have a shaver adaptor for 3 pin sockets but my multi-meter is in work so I can't test anything to see what's wrong. The adaptor only has a 1amp fuse but I wouldn't' have thought the motor would pull enough current to blow the fuse.

Is there any problem with cutting the two pin plug off and fitting a three pin? I'm guessing that as it has two pins there is no earth in the cable, will this affect whether it will work or not or more importantly have any more safety implications than using the adaptor if I fit the three pin plug.

 

[fracster]

No problem, the two pin plug has no earth pin (Generally from outside the UK),the three pin does (UK standard), all appliances used to be earthed in the UK,newer ones don`t need earthing. It should be fine if you cut off the two pin and rewire onto a three pin,as long as you wire it correctly.

Disclaimer......I`m not an electrician, but my father has been all his life and it is something I picked up off him.

 

[Kev M]

Cheers, Fracster, that's this afternoons project sorted.

 

[Paulmack]

BEWARE!!

If it has a european plug top on it and it was manufactured more than say 5 years ago then the colours of the cable may not be what you are expecting. Standard Brown - Live and Blue - Neutral may in fact be White - Live and Black Neutral as I have witnessed on old German motors.

If you are unsure then take the casing off the item and check the connections at the other end.

When you fit the standard UK 3 pin plug I would put a 3A fuse in there too instead of the standard 13A that you will get just to make sure. I doubt it will need more than 3A so its best to be safe as you probably wont easily get your hands on a 1A fuse anyway.

Any problems ask away.

 

[Gofer]

Which wire is connected to what is of no consequence though is it. The unit was originally fitted with a two pin plug that could be plugged in ether way around so there was no guarantee which wire was going to be connected to the phase and which was going to be connected to the neutral anyway.

 

[Paulmack]

Which wire is connected to what is of no consequence though is it. The unit was originally fitted with a two pin plug that could be plugged in ether way around so there was no guarantee which wire was going to be connected to the phase and which was going to be connected to the neutral anyway.

Depends on the kit. If it is a light then no problem really but if it is a motor then yes.

I take your point about the plug being able to be reversed but in the UK all plugtops must be fused on the live side so thats the single biggest reason for making sure polarity is correct.

Dangerous...maybe not...Good Practice...Definately!!

 

[Gofer]

Depends on the kit. If it is a light then no problem really but if it is a motor then yes.

I take your point about the plug being able to be reversed but in the UK all plugtops must be fused on the live side so thats the single biggest reason for making sure polarity is correct.

Dangerous...maybe not...Good Practice...Definately!!

Yes, but my point is the unit Kev M has is fitted with a two pin plug so obviously the design of the unit he has is such that it is not important which wire is connected to the phase. In his case there is no reason why he has to bother which wire goes where just providing he doesn't connect either of then to the earth terminal. Incidentally, a simple single phase AC motor circuit is not phase dependant and will run equally well regardless of how it is connected. One of the main reasons colour coding should be observed is because if any switching arrangement is installed on a device, it will be wired to the live or phase side of the supply and will therefore not only isolate the device when switched off, but add additional protection to the user.

You are of course correct in saying that in the UK the live side of a plugtop must be fused and that feature is designed into each and every plugtop available today, but it's been designed that way simply to ensure that the 'live' or 'phase' is removed from the appliance being protected should a fault occur and the fuse blow and this will happen regardless of how the wiring from the appliance to the plug has been connected.

I agree entirely it's good practise to ensure any plug top is wired properly with the colour coding strictly observed - where applicable.

 

[mrgubby]

If it has a two pin plug AND bought from ebay it may be a US model which is designed to run from 110v. This is why it would pop the fuse in the adapator ....

 

[Gofer]

If it has a two pin plug AND bought from ebay it may be a US model which is designed to run from 110v. This is why it would pop the fuse in the adapator ....

............a very good point (groan) you make there mrgrubby

 

[sturs]

1st of all check the motor to see if it is compatable with 240V 60Hz. If not get yourself a 240v to 110v converter. Then look into the wiring as other have said, if you are still not sure then buzz the earth out to the casing of the motor so you know what each wire is.

Many converters areonly fitted with a small amp fuse i.e. 3 amp. This should be fine with your application but you may find that after a long period of use the converter will get hot and turn off with its own thermal switch.

The only reason I know the above is because I am married to a American and also lived in Canada for many years which ment we brought alot of items back with us and still get gifts sent from the USA. I must have more 240 -110V converters in my house than aybody else

If all the above fails then you can always take the motor to be re-wind, should only cost you about £25

 

[admirable]

I once had a Triumph Stag that had an electrical Problem,............conrod came through the block and knocked the alternator off!


OK, I'll get me coat!

 

[donut]

i should think it had a 1 amp fuse for a reason , so i'd be looking to get one inline somewhere / somehow .

 

[coljun]

You really must estabish if it is a 230v ac motor before you apply uk mains voltage to it. If it is 110v ac or indeed anything else you will knack it.Is there a plate on it showing votage current etc.

 

[Kev M]

It's defo 240v. I played with it last night and was gitting a high resistance in the cable so chopped the plug off and fitted a 3-pin. Bobs your uncle and the motor spins, slowly and with very little torque but that's for a different thread.

Thanks for the help and advice everyone.

 

[russdaz]

No problem, the two pin plug has no earth pin (Generally from outside the UK),the three pin does (UK standard), all appliances used to be earthed in the UK,newer ones don`t need earthing. It should be fine if you cut off the two pin and rewire onto a three pin,as long as you wire it correctly.

Disclaimer......I`m not an electrician, but my father has been all his life and it is something I picked up off him.

Not strictly true, the earthing requirement is totally dependant on the appliance's construction. If the item is classed as being constructed as being double insulated then no earth, if it isnt then it will most deffo will need earthing.

Being double insulated means you cant access live parts, or parts that could become live under fault conditions, eg a plastic cased microwave may not need an earth, a metal cased microwave will most likely need an earth.

American and Europe run different reg's for there voltage, fault and earthing requirements hence there different plug design's. Europe is now much closer to our reg's, so not old european stuff will work correctly over here, due to frequancy differences but be anything from will not work fullstop without a converter due to the difference in voltage. America use 120v which is deamed to prefectly safe to touch, without causing injury, wouldnt reconmend anyone testing that though who dosnt know what there doing.

 

[who?]

It isnt the voltage that kills - It is the current. I've had shocks of thousands of volts - but low current.

On the whole fuses are there to protect the wiring. A fault condition which results in a short can generate very high currents which can melt cables. The fuse blows first. One of the reasons that europeans are less fussed about fuses is that they use radial wiring rather than rings and their switches and mcbs tend to cut both live and neutral.

edit - disclaimer; I am not an electrician either.

 

[russdaz]

It isnt the voltage that kills - It is the current. I've had shocks of thousands of volts - but low current.

On the whole fuses are there to protect the wiring. A fault condition which results in a short can generate very high currents which can melt cables. The fuse blows first. One of the reasons that Europeans are less fussed about fuses is that they use radial wiring rather than rings and their switches and mcbs tend to cut both live and neutral.

edit - disclaimer; I am not an electrician either.

Id remember your disclaimer yourself mate, and stear away from something you obersly do not understand before you kill yourself, the next shock maybe your fatal one. Under the right conditions 230v is lethal, it takes only 200mA to kill.

Hate getting into these posts as i struggle explaining things in text but here goes.

Fuses within the house hold do 2 jobs, protect against overloads to protect the cable (yes), and equally to protect against short circuit faults. That is why we have long winded cable calculations to carry out when designing cable runs to insure the cable can handle the voltage and fault currents that have to be massive to get disconnections times required.

MCBs are generally just more sensitive resetable fuses, and generally single pole i.e switch the phase conductor only. Yes European countries tend to run radials rather then ring main's, but that has b****r all to do with fusing and earthing requirements, infact id love to know any reason why running a radial rather the ring would make you lessed fused by fusing. Infact some industrial circuits here are run as radials, radial circuits need much larger cable to carry the voltage and current over rings. A properly designed circuit running fuses, is just as safe as one running MCB's. MCB's are prefered due to better control of inrush currents.

The main reason Euopean regs may seem easier on fusing is there use on 30mA RCD's for supplementry protection, with the latest 17th Edition regs in this country we are almost at the stage where every cicruit will need RCD, and MCB/fuse protection. Notice i left fuse in there as large currents then fuses are still used or large hydraulic MCB's. The current regs have tighted the belt quite alot mainly to prevent idiots getting themselfs killed from not understanding what they doing.

For those interested that may not know RCD detect a difference (R= Residual) current (C =current) (D= Device) a slight difference between the netraul and phase conductors then trip will be activated, but RCD's supply no fault protect, hence needing MCB's or fuses. A 30mA RCD will trip at a 30mA difference, which is way below the lethal 200mA level

Oh putting to much voltage though a cable can cause it heat up, and melt.

 

[who?]

Id remember your disclaimer yourself mate, and stear away from something you obersly do not understand before you kill yourself, the next shock maybe your fatal one. Under the right conditions 230v is lethal, it takes only 200mA to kill.
Exactly my point - it is current that kills. I was objecting to the 120V is safe, 240V isnt bit and trying to make the point that it is the current which matters rather than the voltage. Any VdGraff generator will throw out huge voltages which are safe - it is the current which kills.

Hate getting into these posts as i struggle explaining things in text but here goes.

Fuses within the house hold do 2 jobs, protect against overloads to protect the cable (yes), and equally to protect against short circuit faults. That is why we have long winded cable calculations to carry out when designing cable runs to insure the cable can handle the voltage and fault currents that have to be massive to get disconnections times required.

MCBs are generally just more sensitive resetable fuses, and generally single pole i.e switch the phase conductor only. Yes European countries tend to run radials rather then ring main's, but that has b****r all to do with fusing and earthing requirements, infact id love to know any reason why running a radial rather the ring would make you lessed fused by fusing. Infact some industrial circuits here are run as radials, radial circuits need much larger cable to carry the voltage and current over rings. A properly designed circuit running fuses, is just as safe as one running MCB's. MCB's are prefered due to better control of inrush currents.
The Europeans seem to use twin pole disconnection on mcbs - The french ones I've used certainly do anyway. And it does have a relevance with fusing (I dont think I mentioned Earths?) your short circuit fault should throw the mcb, but I would argue that mcbs are better because of the response time. A short circuit is anyway just a specific overload condition, isnt it?
I would certainly prefer twin pole disconnect.

As I understand it mcbs have better accuracy and speed response than fuses.

I suppose that the point about radials its that thaey have many more circuits (because of the limits on number of connections per circuit) and seem to have better control of the current levels on the circuits, rather than just slapping big fuses on rings. So in effect the mcb on the radial acts rather like the fuses on appliances that we have in the uk.

I suppose that could also be because they use 3 phase in domestic situations rather more than we do in the uk and have to take account of balancing the phases.

I would argue that rings are just a bodge to reduce cable size.

The main reason Euopean regs may seem easier on fusing is there use on 30mA RCD's for supplementry protection, with the latest 17th Edition regs in this country we are almost at the stage where every cicruit will need RCD, and MCB/fuse protection. Notice i left fuse in there as large currents then fuses are still used or large hydraulic MCB's. In domestic use? The current regs have tighted the belt quite alot mainly to prevent idiots getting themselfs killed from not understanding what they doing.

For those interested that may not know RCD detect a difference (R= Residual) current (C =current) (D= Device) a slight difference between the netraul and phase conductors then trip will be activated, but RCD's supply no fault protect, hence needing MCB's or fuses. A 30mA RCD will trip at a 30mA difference, which is way below the lethal 200mA level

Oh putting to much voltage current though a cable can cause it heat up, and melt.

I dont think that we are disagreeing as much as you think we are - I probably didnt express myself too well though in my original post.

I am all in favour of mcb/rcd/rcbo protection. I've not read the 17th edn, but I was told that they were going to start and introduce radial circuits into the uk regs. Did it happen?

 

[russdaz]

I dont think that we are disagreeing as much as you think we are - I probably didnt express myself too well though in my original post.

I am all in favour of mcb/rcd/rcbo protection. I've not read the 17th edn, but I was told that they were going to start and introduce radial circuits into the uk regs. Did it happen?

Couldnt work out how to fully quote, but to answer a couple of things.

110v is deemed safe, due to safe touch voltages which our regulations are designed to which means the current shouldnt go above the safe current. Its why 110v is used on building sites. Any voltage can be danagerious as it can causes burns, hence why i wouldnt reconmend touching live cable unless you know the conditions.

But there situations where you make the hole work space live meaning anything you touch is live at what ever voltage.

The European's (Germany i heard where going more our way, but they are different France) are going single pole MCB's more these days backed by Double pole RCD's, dont get to het up about double isolation for most domestic uses in the UK, is a biggy in parts of Europe due to twin plug rather then 3pin UK. But no a MCB will not trip faster then a correctly designed fuse. The bennifit from MCB's is you can use smaller cables, or extended cable runs, and they control inrush currents better, and a reusable. Inrush currents are pretty much redunadant in the domestic environment.

The argument for radials in these country is pretty poor tbh, yes under 3 phase supplies may be of bennfit to help prevent 415 potential. Rings are far more versitle and easily expandable if required where radials are not. a 500amp ring is just as safe as a 10amp radial, provided both are constructed in the correct manor.
The reason why rings are being considered to be withdrawn for the UK regs, is numbty DIY'er's who think there electrictions and know better, and expand the ring incorrectly or even split the ring into 2 radials.

Hydralic MCB's are not used in domestic environment's, or least i have never seen them yet, but 100- 400amp fuses are still used in some circumstances.

One other thing to clear up to higher a voltage though a too smaller a cable causes increased resistance and voltage drop, which will increase current yes, but that increase was purely down to the higher voltage.

Personally i am all for the lastest update, RCBO's are the way to go IMHO as you gain circuit discrimination, but again tbh RCBO's will only be safe provided they are installed correctly and not abused, but theres to abuse on them then fuses.

 

[Splog]

I think all this talk of safe voltages etc is a bit worrying
IMHO electricity can kill very easily and in very simplistic terms look at it this way:

Volts are pumping power and amps are 'how much' if you reduce the voltage you increase the amps required, so if you decide to touch a 110v supply you will get more but slower!

Cable sizes need to increase and shorten with lower voltages! the equipment pulls what it needs! so if it's only being supplied at 110v and needs 20A it needs a bigger cable than 220v at 10A (can't be arsed with the maths just now)
or at least that's how I remember it

Steve

 

[Gofer]

.................One other thing to clear up to higher a voltage though a too smaller a cable causes increased resistance and voltage drop, which will increase current yes, but that increase was purely down to the higher voltage...................

This is simply not true. The resistance of any cable is decided by the material used and the size of the conductor. It is a constant and does not change regardless of the applied voltage (although changes of temperature will have some effect on the resistance). Volt drop suffered in any cable is a produce of the current flowing and the resistance of the cable. One of the reasons distribution of electricity (pylons etc) is at such high voltage is because by using high voltages the current required to achieve the same power will be much lower, hence less heat and less volt drop (and smaller conductors can therefore be used).

 

[mr_pants]

110v is deemed safe, due to safe touch voltages which our regulations are designed to which means the current shouldnt go above the safe current. Its why 110v is used on building sites. Any voltage can be danagerious as it can causes burns, hence why i wouldnt reconmend touching live cable unless you know the conditions.

... which is 55V to earth, after all, although it can be shown that in wet conditions, it is possible to drive enough current through a person's body to kill them.

Hydralic MCB's are not used in domestic environment's, or least i have never seen them yet, but 100- 400amp fuses are still used in some circumstances.

Not yet heard about hydraulic MCBs or even MCCBs. There are still 800A fuses being installed in some locations FWIW.

 

[russdaz]

I think all this talk of safe voltages etc is a bit worrying
IMHO electricity can kill very easily and in very simplistic terms look at it this way:

Volts are pumping power and amps are 'how much' if you reduce the voltage you increase the amps required, so if you decide to touch a 110v supply you will get more but slower!

Cable sizes need to increase and shorten with lower voltages! the equipment pulls what it needs! so if it's only being supplied at 110v and needs 20A it needs a bigger cable than 220v at 10A (can't be arsed with the maths just now)
or at least that's how I remember it

Steve

Yes thats very true, and why i have said before i wouldnt risk touching any voltage. But something to remember ishow 110v is produced and supplied in the UK. CTE transformers primarily.

This is simply not true. The resistance of any cable is decided by the material used and the size of the conductor. It is a constant and does not change regardless of the applied voltage (although changes of temperature will have some effect on the resistance). Volt drop suffered in any cable is a produce of the current flowing and the resistance of the cable. One of the reasons distribution of electricity (pylons etc) is at such high voltage is because by using high voltages the current required to achieve the same power will be much lower, hence less heat and less volt drop (and smaller conductors can therefore be used).

Not strictly true, but what i said wasnt stictly true, but was trying to keep it simple, but multi-core cables are designed with dieletrics (SP) which effects in mind for a given voltage range, that figure changes with voltage, increased voltage will increasing heat, thus increasing resistance thus increasing current.

... which is 55V to earth, after all, although it can be shown that in wet conditions, it is possible to drive enough current through a person's body to kill them.




Not yet heard about hydraulic MCBs or even MCCBs. There are still 800A fuses being installed in some locations FWIW.

Your correct on your voltage, and sure i neednt go into the explaination of its releanance, particularly to how 110v is produced.

Not seen 800A domestically highest i have come accross is 400A, Hydraulic MCB's are used for remote isolation, and monitoring perposes, primarily major networks, under fault they act as normally MCB's but canbe reset remotely, or can be used to isolate supplies remotely under conditions that would have been danagerious to do by hand..

Bit of subject now though.

 

[Gofer]

..........................

Not strictly true, but what i said wasnt stictly true, but was trying to keep it simple, but multi-core cables are designed with dieletrics (SP) which effects in mind for a given voltage range, that figure changes with voltage, increased voltage will increasing heat, thus increasing resistance thus increasing current. .....................

Nope, an increase in resistance will most certainly not increase current. I'm not quite sure what you're getting at with your reference to dielectrics but as far as cables are concerned the dielectric simply is the insulation material between the cables and yes, if you exceed the rated voltage of any multicore cable you do run the risk of the insulation breaking down. Obviously once the insulation has broken down current will flow between conductors and if this current is not arrested heat will be produced, but any increase in resistance as a result of that heat will not increase current flow however.

Oh, just for the record, the comments I made earlier where absolutely true.

 

[bsmotorsport]

but any increase in resistance as a result of that heat will not increase current flow however.

so where does the heat come from? you dont get anything for nothing in this world, if current is been used to generate the heat and the load is still using the same current, together it HAS to be pulling more current.

 

[MattF]

so where does the heat come from? you dont get anything for nothing in this world, if current is been used to generate the heat and the load is still using the same current, together it HAS to be pulling more current.

Heat is generated as the moving electrons hit the stationary atoms within a metal lattice. The heat comes from atom vibration.

 

[bsmotorsport]

which has to be using energy does it not? energy supplied by an increase in current flow, unless you've just struck on a new way of getting free heat energy

 

[mr_pants]

which has to be using energy does it not? energy supplied by an increase in current flow, unless you've just struck on a new way of getting free heat energy

Basically what happens is that there is energy dissipated in the cable and in the load. Simply put, the energy that's flowing into the system is dissipated within the system as a whole. This means that, in effect, the more losses in the cables, the less energy there is available at the load. The resistance of the cables does not rise terribly significantly, as they normally don't get anywhere near hot enough. The IR losses in the cables mean that the voltage actually appearing across the load is reduced, even though the current is still the same (current is common in a series circuit) hence the energy dissipated in the load is lower.

 

[mr_pants]

Not seen 800A domestically highest i have come accross is 400A, Hydraulic MCB's are used for remote isolation, and monitoring perposes, primarily major networks, under fault they act as normally MCB's but canbe reset remotely, or can be used to isolate supplies remotely under conditions that would have been danagerious to do by hand..

Bit of subject now though.

I can't imagine 400A devices being used domestically. I've still never heard of hydraulic MCB's. Are you sure you're not confusing it with pneumatically-operated devices, such as air-blast circuit breakers, which historically have been used in the super grid.

The normal types of remotely-operated circuit breakers for distribution would be motor-driven or stored-charge, as solenoid-operated is now largely superseded.

I think we've gone comprehensively OT now.

 

[bsmotorsport]

Basically what happens is that there is energy dissipated in the cable and in the load. Simply put, the energy that's flowing into the system is dissipated within the system as a whole. This means that, in effect, the more losses in the cables, the less energy there is available at the load. The resistance of the cables does not rise terribly significantly, as they normally don't get anywhere near hot enough. The IR losses in the cables mean that the voltage actually appearing across the load is reduced, even though the current is still the same (current is common in a series circuit) hence the energy dissipated in the load is lower.

I see where your coming from but, as the voltage across the load is reduced the current would therefore have to increase to supply the specific VA needs of the device, as this is mains we are talking about bar the physical limits of the cable we have an unrestricted current supply. The current would rise, creating more heat in the cable, as it gets hotter the resistance goes up, more losses and greater current draw and so on and so on.
But yes WAY OFF TOPIC. appologies to the original poster

 

[russdaz]

Nope, an increase in resistance will most certainly not increase current. I'm not quite sure what you're getting at with your reference to dielectrics but as far as cables are concerned the dielectric simply is the insulation material between the cables and yes, if you exceed the rated voltage of any multicore cable you do run the risk of the insulation breaking down. Obviously once the insulation has broken down current will flow between conductors and if this current is not arrested heat will be produced, but any increase in resistance as a result of that heat will not increase current flow however.

Oh, just for the record, the comments I made earlier where absolutely true.

Have you taken into account the eddys' that would exist as the insulation cant contain the energy field, eddy current do increase heat and resistance, and can effect single phase cable at high enough voltages. WHich is from i what have heard is the princible behind cable thumpers that output high voltages and low currents i.e 25kV and 400mA, but the guidelines is to thump at the lowest voltage so as not to damage the rest of the cables insulation. A thumped cable is more prone to more failures after, particularly after quite a big thumping sessions.

I can't imagine 400A devices being used domestically. I've still never heard of hydraulic MCB's. Are you sure you're not confusing it with pneumatically-operated devices, such as air-blast circuit breakers, which historically have been used in the super grid.

The normal types of remotely-operated circuit breakers for distribution would be motor-driven or stored-charge, as solenoid-operated is now largely superseded.

I think we've gone comprehensively OT now.

No mate i know the types you mean, i have only seen these types used in one situation on a private network system before, but they are new, atleast to these country as far as i know. There a Chinese design I dont think there replace the types you are refering to though as these where only rated for 250v.

I read the Chinese have started a while back making hydraulic Magnet operated MCB's using hydraulic instead of the bimetalic strips on which these where developed.
Must admit i havent seen them being available within this Country, i cannot for the life of me remeber the company (Chinese) that made them, and i am no-longer working on them as i have changed job, but i shall try get some more info on them from my mate, who hadnt seen them before either, but have found out these types are availalbe in standard MCB ratings.

Agree though we way off original

 

[mr_pants]

WHich is from i what have heard is the princible behind cable thumpers that output high voltages and low currents i.e 25kV and 400mA, but the guidelines is to thump at the lowest voltage so as not to damage the rest of the cables insulation. A thumped cable is more prone to more failures after, particularly after quite a big thumping sessions.

Capacitor discharge units (surge generators, 'thumpers' if you will) output a current pulse, due to the capacitor discharging into the fault, even though the high voltage source charging the capacitor is outputting at a low amperage.

I agree that logic would dictate that a thumped cable is more prone to failure and have always held to that tenet, although I've never come across an example where thumping has actually caused a subsequent failure. Indeed, I regularly thump LV cables at up to 6kV, simply to get the fault to make enough noise.

 

[mr_pants]

I see where your coming from but, as the voltage across the load is reduced the current would therefore have to increase to supply the specific VA needs of the device, as this is mains we are talking about bar the physical limits of the cable we have an unrestricted current supply. The current would rise, creating more heat in the cable, as it gets hotter the resistance goes up, more losses and greater current draw and so on and so on.
But yes WAY OFF TOPIC. appologies to the original poster

It depends on the device as to what would happen. If it's something like a heater, then the action of the reduced voltage and, nominally, same current,will lead to less energy being dissipated in the device (the VA product being less). Consider the case of people in rural areas with long lengths of overhead low-voltage lines, where the lights run dimly. If it's a motor, then it would run a little slower and actually cause more current to be drawn overall. This would cause slightly more volt-drop in the cables,but, simply put, the whole system would end up in equilibrium.

 

[russdaz]

Capacitor discharge units (surge generators, 'thumpers' if you will) output a current pulse, due to the capacitor discharging into the fault, even though the high voltage source charging the capacitor is outputting at a low amperage.

I agree that logic would dictate that a thumped cable is more prone to failure and have always held to that tenet, although I've never come across an example where thumping has actually caused a subsequent failure. Indeed, I regularly thump LV cables at up to 6kV, simply to get the fault to make enough noise.

LV cables, and assoated fuses MCB's etc are rated to withstand 6kV for a few seconds under fault condition, cant remember the dates that was introduced, but before i think it was 5kV? There was article on the IEEE website that stuided thumpers and did prove there is an encrease of faulires once thumped on the cable, try running 12kV and feel the heat in the cable after. Think most thumpers are about 4ms bursts of power, so the damaged caused to the LV network at 6kV should be minimal. I have been on a fault location on a HV cable that took some time thumping to find, and was quite badly damaged, and refaulted soon after.

Those types of thumpers and test talked of above is pretty new to me tbh, but the effects of over voltage, or high voltage testing isnt.

Interesting link here with a couple of videos that explain things like Dielectric Failure alot better then i could, although dosnt give any info on eddy currents, thats if any-one else is interested ofcourse lol

http://www.cirris.com/testing/guidelines/hipot_testing.html

 

[mr_pants]

LV cables, and assoated fuses MCB's etc are rated to withstand 6kV for a few seconds under fault condition, cant remember the dates that was introduced, but before i think it was 5kV? There was article on the IEEE website that stuided thumpers and did prove there is an encrease of faulires once thumped on the cable, try running 12kV and feel the heat in the cable after. Think most thumpers are about 4ms bursts of power, so the damaged caused to the LV network at 6kV should be minimal. I have been on a fault location on a HV cable that took some time thumping to find, and was quite badly damaged, and refaulted soon after.

Those types of thumpers and test talked of above is pretty new to me tbh, but the effects of over voltage, or high voltage testing isnt.

Interesting link here with a couple of videos that explain things like Dielectric Failure alot better then i could, although dosnt give any info on eddy currents, thats if any-one else is interested ofcourse lol

http://www.cirris.com/testing/guidelines/hipot_testing.html

The question I have to ask is whether the cable was, indeed, damaged by the capacitor disharge test or by the initial in-service failure? The energy delivered from a thumper is much less than that delivered by a system fault by several orders of magnitude.

 

[bsmotorsport]

Intresting one for you lads, approx 2 weeks ago I finished commissioning an MV switchboard at Heathrow, well yesterday some kind civil engineer decided to put his machine mounted pecker through one of the outgoing cables. We know where the fault is but the cable was only a temp for a few months so its going to be left as it is as it was part of a ring they've just opened it and running as two radials for now. Anyway, I was asked to download the recording from the relay that cleared the fault on the MV board that we had put in. The fault occured between l2 and l3 and peaked at 10,900 amps, which at 11,000V is a serious ammount of energy. It was cleared within 800ms but christ I wouldnt have liked to have been near that one! No-one was hurt apart from the baggage hall which lost power briefly and 4 planes had to leave with no baggage!!!