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Discuss RCD tripping using test button, N-E earth fault. in the Electrical Forum area at ElectriciansForums.net

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Kev2632

Regular EF Member
Hello, ive just been at a job today which i will be returning tomorrow which i have a problem with 2 Ring mains on the RCD,

*1 RINGMAIN 1
*2 RINGMAIN 2

Now when i switch on the RCD, and NO mcb's are on, the Test button doesnt work, and its the same when the mcb's are switched,

Then i tried disconnecting 1 of the neutrals from RINGMAIN 1, and put the RCD back on, the test button now works?

Why would this be happening? and is it a N-E fault, if so why doesnt the RCD trip straight away when the power is put onto the Circuits?
 

cook1e

Regular EF Member
Have you tested it properly with a tester, have you mixed up the supplied neutrals, does your meter show a neutral eth fault ?

Martin
 

telectrix

Scouser and Proud of It
Respected Member
N-E faults can sometimes cause weird effects with RCDs. get the IR tester on the job.
 

Kev2632

Regular EF Member
no lads, havent had a chance to megger it yet, but i'm just wondering as to why the RCD Wont trip with that Neutral connected, as soon as i take that 1 leg of the neutral out its trips no problem with the test button?? :/
 

Kev2632

Regular EF Member
wylex board, with a TNC-S, But im trying to get my head round this"As to why the RCD Wont trip with that Neutral connected, as soon as i take that 1 leg of the neutral out its trips no problem with the test button?? :/"
 

telectrix

Scouser and Proud of It
Respected Member
similar. a N-E fault on 1 RCD in a dual RCD board can cause the other RCD to trip. just a question of dissing all the neutrals, unplugging all appliances, then IR testing each one.
 

Kev2632

Regular EF Member
similar. a N-E fault on 1 RCD in a dual RCD board can cause the other RCD to trip. just a question of dissing all the neutrals, unplugging all appliances, then IR testing each one.
So basically ive gotta get into my head that RCD's can be a right pain the a**e then basically? and the N-E fault will also cause the RCD test button to not trip, if the test button is used? but why is that do you think? why didnt the RCD TRIP straight away when power was put onto the circuits????
 

Kev2632

Regular EF Member
Found this..... could this sum up my problem a little bit?????

1/ All MCB's off so even if power is on to the supply side of the RCD, there is no current flowing through the RCD yet cuz no loads switched on?

No current flowing so RCD wont trip even with a N-E fault present.


2/ If you had got some of your MCB's on with some appliances or lights switched on somewhere in the property I suspect the RCD would have tripped cuz current could now bypass the N half of the RCD through your N-E fault.


3/ Is the supply on the installation PME/TN-C-S..
i.e. Neutral and Earth joined together at the suppliers main cut-out?
cuz this will give you the symptoms you are describing:-

I will try & explain:-

Have a look at figure 11.1 on page 93 of the On-Site-Guide.

IF PME/TN-C-S, you can draw an connection between the N & E on the bottom left hand side of the drawing..

Now also put another connection between the N and E just before the load on the bottom right hand side of the drawing. (your N-E fault)

Remember the brown live wire from the RCD to the load would be open circuit because your MCB's are off..


Now lets consider a possible test current path...

  • From Live
  • Up the black wire to the test button
  • Through the test button
  • Through the test trip load resistor
(This is where it can now take two paths)
  • (i) Along the neutral through the RCD coil

    or
  • (ii) Along the neutral wire toward the load
  • (ii) Through your N-E fault
  • (ii) Along the Earth wire
  • (ii) Back from the Earth to the Neutral at the PME/TN-C-S connection.
If this is a PME/TN-C-S earth arrangement the two paths will probably be of a very similar resistance so the fault test current will most likely divide equally down the 'N' and the 'E'.

So the little bit of current flowing through the N coil of the RCD is probably too small to cause it to trip!


If Earth arrangement is TN-S or TT the resistance via your N-E fault down the earth conductor will be higher so the test fault current going down the E will be less..

So more will go down the N coil of the RCD and provide enough to trip the RCD with the test button!
 

leonard

Regular EF Member
So basically ive gotta get into my head that RCD's can be a right pain the a**e then basically? and the N-E fault will also cause the RCD test button to not trip, if the test button is used? but why is that do you think? why didnt the RCD TRIP straight away when power was put onto the circuits????
Kev the RCD test button is the last test you have to do after you test continuity of protective conductors,insulation resistance,polarity,Zs,RCD test +/_ Cycle and then pres the test button by using proper instruments and that will clear your mind.
 

Kev2632

Regular EF Member
Kev the RCD test button is the last test you have to do after you test continuity of protective conductors,insulation resistance,polarity,Zs,RCD test +/_ Cycle and then pres the test button by using proper instruments and that will clear your mind.
yea i know all that, i will be doing that tomorrow!, but im just trying to fathom out why the rcd wont trip with the test button at the moment,

does this sum it up....????
1/ All MCB's off so even if power is on to the supply side of the RCD, there is no current flowing through the RCD yet cuz no loads switched on?

No current flowing so RCD wont trip even with a N-E fault present.


2/ If you had got some of your MCB's on with some appliances or lights switched on somewhere in the property I suspect theRCD would have tripped cuz current could now bypass the N half of the RCD through your N-E fault.


3/ Is the supply on the installation PME/TN-C-S..
i.e. Neutral and Earth joined together at the suppliers main cut-out?
cuz this will give you the symptoms you are describing:-

I will try & explain:-

Have a look at figure 11.1 on page 93 of the On-Site-Guide.

IF PME/TN-C-S, you can draw an connection between the N & E on the bottom left hand side of the drawing..

Now also put another connection between the N and E just before the load on the bottom right hand side of the drawing. (your N-E fault)

Remember the brown live wire from the RCD to the load would be open circuit because your MCB's are off..


Now lets consider a possible test current path...

  • From Live
  • Up the black wire to the test button
  • Through the test button
  • Through the test trip load resistor
(This is where it can now take two paths)
  • (i) Along the neutral through the RCD coil

    or
  • (ii) Along the neutral wire toward the load
  • (ii) Through your N-E fault
  • (ii) Along the Earth wire
  • (ii) Back from the Earth to the Neutral at the PME/TN-C-S connection.
If this is a PME/TN-C-S earth arrangement the two paths will probably be of a very similar resistance so the fault test current will most likely divide equally down the 'N' and the 'E'.

So the little bit of current flowing through the N coil of the RCD is probably too small to cause it to trip!


If Earth arrangement is TN-S or TT the resistance via your N-E fault down the earth conductor will be higher so the test fault current going down the E will be less..

So more will go down the N coil of the RCD and provide enough to trip the RCD with the test button!
 

needasparks

Electrician's Arms
Does sound about right to me, Rcd's do funny things when a N-E fault is present and now you mention it wylex have done that to me on the last two i've tested. It won't trip on test button but will go randomly but when you test at x1 you will get a very fast trip time like less than 10ms. Any overly fast times normally that you would not expect to get usually ends up a N-E fault.
Am sure your IR test will confirm as tel says.
 
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