RCD earth potential

[Stringy75]

Ok the external fault loop impedance. The earth conductor, the transformer and the incoming line conductor.

 

[steptoe]

Ok the external fault loop impedance. The earth conductor, the transformer and the incoming line conductor.

IMPEDANCE is the magic word

its a world away from resistance

 

[Stringy75]

The mere fact that your tester will not do the test, should indicate to you that something is wrong, fortunately with the tester indicating the problem you do not need to test for polarity unless you wish to confirm the testers indications.




 

I now see the importance of following the test sequence, and I now understand what people were trying to tell me all along. I feel very foolish. Thank you for explaining this to me in such an instructive manner

 

[Stringy75]

IMPEDANCE is the magic word

its a world away from resistance




 

Sorry you are correct, impedance accounts for capacitance and inductance and is the correct word to use when talking about AC circuits

 

[Sharpend]

Ok so now we have your focus, please review the posts and establish the guidance you were given. Do you have a GN3? If so it would be worthwhile refreshing your memory on its contents, this is key to your development. You may even now be able to enlighten the two that you work with with your new found learning.

 

[steptoe]

Never ever overestimate the basics,

I'd say >80% of the faults I attend after others are very very basic problems that previous sparks had overlooked in their wisdom to find that complicated issue

 

[Stringy75]

Ok so now we have your focus, please review the posts and establish the guidance you were given. Do you have a GN3? If so it would be worthwhile refreshing your memory on its contents, this is key to your development. You may even now be able to enlighten the two that you work with with your new found learning.




 




 

Yes I have a copy of guidance note 3, I have read it. I will re-read it again too. Maybe the electricians I am working with are not good at fault finding. They are good electricians though

Never ever overestimate the basics,

I'd say >80% of the faults I attend after others are very very basic problems that previous sparks had overlooked in their wisdom to find that complicated issue




 




 

I am glad i joined this forum. Started me off on a good foot. I will follow the testing procedure by the word in future at the start off any fault finding job and go from there

 

[Stringy75]

One question, if a nail did bridge line and earth, would earth gain the same potential as the line?

 

[Doc Hudson]

I now see the importance of following the test sequence, and I now understand what people were trying to tell me all along. I feel very foolish. Thank you for explaining this to me in such an instructive manner

Yes I have a copy of guidance note 3, I have read it. I will re-read it again too. Maybe the electricians I am working with are not good at fault finding. They are good electricians though

Stringy75, without wishing to mince words, for you to admit that you feel foolish takes some balls!  And I will say without fear of embarrassment or contradiction that this forum really does want to help anyone who is asking a genuine problem and is also willing to listen. Possibly a stumbling block in your understanding of this problem is that C&G courses cover Installation, Understanding of Regs, Inspection & testing. But there is no standard C&G course to cover good practice at fault finding.

One key bit of guidance I would say to you is this:-  Electricity can kill a healthy adult in less than half a second. The fundamental principal that 99% of installations that we come across in the UK is by using ADS (automatic disconnection) to remove the power in the event of a fault.,This relies upon low earth fault loop impedance paths, good earthing and bonding and correct polarity. So any new job you are working on, be it installing, inspecting or fault finding,  you want to be sure that the Earth, bonding, polarity, Ze are all good.  As it could save your life not just your customer.

Almost always the first thing you need to find on any job is the location of the CU / Distribution board.  While identifying the isolation points for any circuits you need to work on, check out that earthing and bonding are good. Verify Polarity while you are checking that your voltage tester is working OK prior to checking any circuit you are about to work on is dead. Think of a possible worst case scenario you could have on your installation such as, all exposed metal parts become live and customer gets a fatal electric shock while you are there. How can you prevent this?  Checking earthing, bonding and polarity is a very good staring point, because if these are wrong MCB's, RCD's & RCBO's, (or the main fuse),  may not do their job correctly.

Also there are quite a few electricians who are the dogs danglies at installing and following wiring diagrams. But they don't have a clue about fault finding or inspection & testing.

Doc H.

 

[Doc Hudson]

One question, if a nail did bridge line and earth, would earth gain the same potential as the line?

Not if the other end of the CPC IS attached to electrical earth. Say 0.35 or 0.8 ohms.  Or imagine you had 2ohms  through live (+nail) to earth,  I=V/R.   230v/2ohms = 115Amps.  (you weren't on a TT installation were you?). Do you not think that a 32A MCB would trip with 100+ Amps flowing?

Doc H

 

[Stringy75]

Stringy75, without wishing to mince words, for you to admit that you feel foolish takes some balls!  And I will say without fear of embarrassment or contradiction that this forum really does want to help anyone who is asking a genuine problem and is also willing to listen. Possibly a stumbling block in your understanding of this problem is that C&G courses cover Installation, Understanding of Regs, Inspection & testing. But there is no standard C&G course to cover good practice at fault finding.

One key bit of guidance I would say to you is this:-  Electricity can kill a healthy adult in less than half a second. The fundamental principal that 99% of installations that we come across in the UK is by using ADS (automatic disconnection) to remove the power in the event of a fault.,This relies upon low earth fault loop impedance paths, good earthing and bonding and correct polarity. So any new job you are working on, be it installing, inspecting or fault finding,  you want to be sure that the Earth, bonding, polarity, Ze are all good.  As it could save your life not just your customer.

Almost always the first thing you need to find on any job is the location of the CU / Distribution board.  While identifying the isolation points for any circuits you need to work on, check out that earthing and bonding are good. Verify Polarity while you are checking that your voltage tester is working OK prior to checking any circuit you are about to work on is dead. Think of a possible worst case scenario you could have on your installation such as, all exposed metal parts become live and customer gets a fatal electric shock while you are there. How can you prevent this?  Checking earthing, bonding and polarity is a very good staring point, because if these are wrong MCB's, RCD's & RCBO's, (or the main fuse),  may not do their job correctly.

Also there are quite a few electricians who are the dogs danglies at installing and following wiring diagrams. But they don't have a clue about fault finding or inspection & testing.

Doc H.




6

Thank you Doc, Thank you for taking the time to write this and for your advise. I will always check  Earth, bonding, polarity and Ze first in future on every installation I work on. Much appreciated

 

[Stringy75]

Not if the other end of the CPC IS attached to electrical earth. Say 0.35 or 0.8 ohms.  Or imagine you had 2ohms  through live (+nail) to earth,  I=V/R.   230v/2ohms = 115Amps.  (you weren't on a TT installation were you?). Do you not think that a 32A MCB would trip with 100+ Amps flowing?

Doc H




 

Not on TT, it was a TN-C-S system. 100 amps would trip an MCB but i don't think it would trip in 0.4 seconds. Would it not need 160A to trip in 0.4 seconds on a type B MCB. 32 x 5 = 160. The bimetallic strip would trigger the trip in a short time, not sure how long it would take the solenoid to trip it though, if at all. So what would happen to the voltage in a short circuit, if earth didn't gain a potential of line, would line drop it's potential to earth, or would both stay at the same potential? Would at the point of the short circuit the potential be half of each and closer to the ends of the conductors be 230 and 0?

 

[binky]

get yourself a 'socket and see' plug in polarity tester, quick easy and reliable. Worth getting the version that makes a noise, very useful for helping isolate a circuit if working on your own.

I've chased similar faults several times, they are very confusing when first encountered, but as stated previously are almost certainly the incoming supply. On 2 occassions it resulted in calling out DNO to dig up road to repair dodgy neutral.

Very glad to see you now recognise the regulars are trying to educate you in their own funny little way.  :Applaud   They like to make youngsters like yourself  think as a way of making you a better electrician. Good luck with the fault finding and do please report back.

 

[ProDave]

Testing Ze does not prove polarity, Ze is the resistance of the incoming earth. Dead testing is not just testing the resistance of the incoming earth. The continuity dead tests prove polarity of the internal circuits  and the polarity live test proves the polarity of the incoming supply. You are really embarrassing yourselves

Please enlighten us. How do YOU test Ze? What instrument? how many probes connected to where? What would your particular tester do if it detected reverse polarity?

I must, like loads of others here, have an "unusual" tester that tells me if the polarity is wrong and won't give a reading.

P.S measuring Ze is not just an earth continuity check from the MET to the CU.

 

 

[Murdoch]

Thinking about it I'm going to stop wasting my time using my expensive MFT and years of experience to do proper tests and revert to the bang test.....or the lick test!

 

[kerching]

Somebody please help me, I do not want to troll through the last 114 posts

why were you called to the installation

what was the reported fault/issue

are all the people involved in this...installers and testers ALL from the same company, if not ( and I think it was mentioned that the install had not been commissioned) why we're different companies involved

what was the actual,fault

all,of these questions may well have been answered previously I was just looking for some précis 

 

[Doc Hudson]

It had been re-wired, the "electrician" who installed it would not go back, some kind of falling out. It is the worst job I have ever seen. Everything is  just chock blocked and not even taped. The guy I am working for was called in because people were getting shocks from sockets, the sockets are not screwed to the back boxes 

No there is no working earth, the earth has a potential of 238V in respect to Neutral

This earlier post and the mention about sockets and shocks is an area that should have highlighted polarity warning signs. Before opening things up I would be wanting a rough idea of what I am looking at, how good the earth is, on the socket reported as giving shocks. A quick plug of polarity tester or my MFT to see what the Zs is at the socket would both have shown warning lights or bleeps to say reversed polarity. The basic error made was in reading 230v between N & E, but not verifying which conductor was at 0v and which was at 230v.  You were most likely led up the wrong path by the other electrician saying the earth was at 230v, then you got fixed in a mindset of looking for the wrong fault.

When fault finding on your own, listen to what symptoms are being reported to you, then go back and verify for yourself exactly what is actually there. Only work on facts that you have established and confirmed for yourself. Treat any verbal claims as items on the list for you to check not factual proven items. When tracing faults on an installation you have never been to, assume its been installed by unskilled untrained DIY friend of a man down the pub with no test equipment. Never assume it must have been right originally, take nothing for granted, until you have proven otherwise assume everything is potentially wrong. Beware of floating conductors showing apparent phantom voltages, especially common when investigating lighting problems. Providing it is feasible with the physical location It can be useful to have a long lead connected back to the MET as a known good 0v earth reference point, as on other occasions you may not have a good local earth to reference any readings to.

Doc H.  

 

[apprentice87]

Why cannot you verify polarity with a simple neon screwdriver

Isolate installation..

then

1, Step one; Stand on the ground

2, Step two, Stick said screw driver on the "phase" conductor.. It lights up?? GOOD!!! It don't???? Step three then..

3, Step three: Stick said screwdriver on the "neutral" conductor.. It lights up?? BAD!!! [Very bad!!]

john..

 

[ProDave]

Why cannot you verify polarity with a simple neon screwdriver

Isolate installation..

then

1, Step one; Stand on the ground

2, Step two, Stick said screw driver on the "phase" conductor.. It lights up?? GOOD!!! It don't???? Step three then..

3, Step three: Stick said screwdriver on the "neutral" conductor.. It lights up?? BAD!!! [Very bad!!]

john..

Very valid, but neon screwdrivers are out of fashion. People are worried about the tiny current that passes through you when they light up, and the infinitely small possibility that if two components in the screwdriver both failed short circuit, that you would then get a real electric shock.

Personally I have a belief that I don't touch anything that my neon screwdriver has not touched first. I have yet to hear of a case of one of the components failing short circuit, let alone two.

 

 

[steptoe]

Why cannot you verify polarity with a simple neon screwdriver

Isolate installation..

then

1, Step one; Stand on the ground

2, Step two, Stick said screw driver on the "phase" conductor.. It lights up?? GOOD!!! It don't???? Step three then..

3, Step three: Stick said screwdriver on the "neutral" conductor.. It lights up?? BAD!!! [Very bad!!]

john..

Sorry to say it John,

But that is typical DIY talk,

neon screwdrivers are brilliant, I wouldn't be without mine,

But to say it could be used in that manner is at best complete misinformed carelessness, 

A little knowledge and all that,

and we know your not a foolish man, but that sort of statement is just totally ridiculous.