Rcd discrimination

[rich.250]

Hi guys.
Called to very strange call out today, cannot get around why and what exactly is going on.
Heres the set up,
TT system with a rod at 51ohms.
100mA s type rcd upfront feeding 2 units. A switch fuse to a submain for the main house db and single 30mA rcd board feeding the rental part.
Basically the fault was discovered while the customer was trying to re heat his tea! Lol
Either microwave will trip the 100mA RCD..... all final circuits are 30mA protected and cant see how this is happening.
RCDs trip at the following when not in series.
100mA S - 70mA, > 2000 @ 1/2 and 356 @ x1
So all ok.
30mA - 26mA, > 2000 @1/2, 56@x1 and 8@x5.
So all ok.
Until they go in series....
Then both trip at 1/2 there rated current. And sometimes when testing the upfront rcd it will tril the 30 after it.
Im so confused, Maybe over thinking it.
Thanks in advance.

 

[spark 68]

You need to IR test and possibly clamp with an Earth leakage meter the various parts/DBs.

It is more than likely cumulative leakage on the 100mA RCD, it could very well be very near it's tripping point so to speak in normal use, if you can break down the individual leakages then you will have a better chance of seeing what is happening.

 

[JD6400]

How many rod positions , and are they electrically separated from each other ?

 

[R G]

There could be some leakage on the circuits already without the additional appliance just taking the 100ma rcd over the limit ,was the ramp test carried out with all circuits live , You could try an earth leakage clamp meter with appliance connected and trace any leakage back

 

[GLENNSPARK]

should be an s type as well...

 

[nickblake]

Series , hope not , possibility a fault on one of the boards especially if its tripping the 100ma insulation test i think

 

[Richard Burns]

Hi guys.

And sometimes when testing the upfront rcd it will trip the 30 after it.

Thanks in advance.

I am slightly worried about this statement, as if you are testing the 100mA S type at the RCD then there is no combination of faults that can trip the 30mA RCD because the "fault" introduced by the tester is upstream of the 30mA RCD so it cannot trip it.
Therefore it seems that there is a wiring error in the sub main supply arrangements, which is odd because it is difficult to do.
The only thing I can think of is that a neutral supply from one board is shared with the other board, but this situation would have tripped things from the start, so what has changed?

 

[Engineer54]

Time to purchase a leakage clamp meter methinks!! Cause something is very wrong here!!

 

[rich.250]

Hi guys. Thanks for the responses. Back on site now. Just to clarify.
61008 100mA s type rcd upfront.
4293 30mA in db.
I have fully tested the installation and no faults. Lowest insulation of around 25Mohms. Most > 500Mohms. Leakage clamp is showing combined leakage ecceeding 70mA which is what the rcd is tripping at.
I have just discovered two more tails that are coming off the back of the tail blocks through the wall into a changeover switch. This then goes to swa across the yard to a generator. At the generator there are more tail blocks and branched off to another changeover switch fed from a seperate service head!!
Both changeover switches appear to be single pole (neutrals have continuity in all combinations). So im thinking this is where my neutral path is, obviously not returning through the 1st rcd.
Thanks for the advice guys, just going to disconnect the generator cables and see if it clears my fault.

 

[Rockingit]

Hi guys. Thanks for the responses. Back on site now. Just to clarify.
61008 100mA s type rcd upfront.
4293 30mA in db.
I have fully tested the installation and no faults. Lowest insulation of around 25Mohms. Most > 500Mohms. Leakage clamp is showing combined leakage ecceeding 70mA which is what the rcd is tripping at.
I have just discovered two more tails that are coming off the back of the tail blocks through the wall into a changeover switch. This then goes to swa across the yard to a generator. At the generator there are more tail blocks and branched off to another changeover switch fed from a seperate service head!!
Both changeover switches appear to be single pole (neutrals have continuity in all combinations). So im thinking this is where my neutral path is, obviously not returning through the 1st rcd.
Thanks for the advice guys, just going to disconnect the generator cables and see if it clears my fault.

So you've THREE sources of energy, combined, with no neutral isolation, and sharing one common point of TT??! (Not of your doing, I realise). Jeez, let's just tear up the regs and make it up as we go along. With any luck we'll only kill the cheap labour.......

 

[rich.250]

It is a complete lash up!!! I have disconnected the generator supply and all is good with regards to tripping. Have noted all the problems and surprise surprise. She doesnt want to do anything about them. Get the impression they have an "electrician" (so called) that installed all of this junk and when a fault arose couldnt sort it. So they called me.
Thanks to all for the help.

 

[dazza1981]

It is a complete lash up!!! I have disconnected the generator supply and all is good with regards to tripping. Have noted all the problems and surprise surprise. She doesnt want to do anything about them. Get the impression they have an "electrician" (so called) that installed all of this junk and when a fault arose couldnt sort it. So they called me.
Thanks to all for the help.

Good luck rich