Unexpected Reading Between Earth And Neutral

[apprentice87]

Not always no, I always check the circuit on its own, then connect it to the earthing bar with all its CPC friends and test again. So far as i know that was one of the changes in the 17th, testing was done with cpc's connected.

Ah, yes,  here we are;

"612.3.1 Insulation resistance...

"The insulation resistance shall be measured between live conductors, and between live conductors and the protective conductor connected to the earthing arrangement. When appropriate during this measurement, line and neutral conductors may be connected together"

john...

 

[not_shocked_yet]

Please, as little abuse as possible appreciated:

I have a 4 gang switch at each end of the room serving 4 sets of 12 v lights, one switch for each set of lights.  For ease I will describe the cabling for one half of the switch plate (i.e. 2 switches as it is mirrored for the other 2 switches.

At the first set of switches inside the entrance to the room there are 3 x twin and earth cables and 2 x 3 core and earth for the switching.  There is a link line between switch 1 and switch 2.

I'm assuming that 1 x twin and earth is supply from the DB.  The other 2 twin and earth cables (one for each switch) must take the supply (when switched) to the lights.  Each 3 core and earth connects the opposite switch at the other end of the room.

At the switch plate at the other end of the room are just the 2 x 3 core and earth cables, one for each switch.

Install was in 2002.

I'm really stuck, brain is hurting.  If you can help me at all it would be much appreciated.

The two core and earth is red and black and (stripped earth with green/yellow sleeve).

So the three twin and earth black neutrals all run outside of the switch between DB and the transformer for the light.  As do the earth's.  One two core and earth red conductor goes to L1 (correct?) - I'll call this one Red 1.  The link line takes the incoming supply from L1 across to L1 on the second switch.  That leaves the remaining twin and earth red conductors that I will call Red 2 and Red 3.    Forget Red 3 as I am just concentrating on the wiring for just one switch and its corresponding switch at the other end of the room.

The 3 core and earth is yellow, blue and red (calling this Red 4).  Forget the second three core and earth as I am just concentrating on the wiring for one switch and its corresponding switch at the other end of the room.

 
From the 3 core the blue (with red sleeve) goes to either L1 or L2 and the red goes to the other - but which way around?.   The yellow (with red sleeve) goes to com on each switch so that's sorted.

The question is:  which way around is the blue and Red 4 and where does Red 2 go?

 

[Andy™]

i cannot understand any of that, my brain is hurting just reading it.

do you have a pic (or even a drawing) of how its connected now, probably easier to understand

 

[not_shocked_yet]

Thanks Andy, this is a picture of the switch plate at one end, there are 10 separate cables in here for the 4 switches but I have brioken it down in my previous post to 5 cables for 2 switches and then just the wiring for one switch.  

If you remember I thought I had fixed the rcd tripping fault because there were 2 loose red conductors when I took the face plate off.  I connected them back where I thought thye should go so this picture might not be wired correctly - if you remember I am getting a 0.05Mohm reading from the lighting circuit indicating a short circuit which is probably caused by me putting the disconnected wires back in the wrong place!  Of course the short could be somewhere else and the wiring could be right.

 

[ProDave]

In general, 2 way switching wired like that will have a red / yellow / blue linking this switch to the remote one. There seems to be no particular standard what colour is used for which.

then feed will go into L1 and out to the light from L2

So you would expect to find one wire in common, in this case the yellow.

You would expect to find (in this case) the red from the 3 core plus one other cable in L1 and the blue from the 3 core plus one other cable in L2

Do start by making sure all 3 cores form ONE 3 core go to one switch correctly, not swapped between switches.

then work out the feeds and outputs to the lights.

 

[not_shocked_yet]

Thanks Pro Dave, does it make any difference (except for the on/off switch position) if the red was in L 2 and the blue in L 1 (ie the other way around?)

Meant to say as long as the corresponding L1 and L2 is used on the remote switch.

 

[ProDave]

They are normally the same at both ends. I can't tell from your switch which is L1 and L2 so I may have them the wrong way round but that makes no difference to function apart from which way is on and off, which can get confusing anyway with 2 way switching.

Get the strappers right first.

Then identify the feeds, and one feed to each switch, then identify the light outputs and one to each switch.

 

[not_shocked_yet]

Cheers Pro Dave, for the inexperienced, wiring of switches (which should be quite obvious in theory) is a bit of a headache!  

 

[Andy™]

looks reasonable straight forward. your other switch will have the 3c&e in the same terminals as this side and ise used for the 2 way switching part. the red going in (thats linked to 2) is the supply in. the other red is the switched supply out to the lights.

basic 2 way wiring really, only real difference here is 2 switches share a common live, rather than an individual live & sl to each light. doesnt make any difference is the red or blue on the 3c&e is the other way, or if the L & SL are the other way

 

[not_shocked_yet]

Pro Dave, can you expand a bit ion yesterday's question, not sure if the answer:

But that's not the end of it.

A N-E fault can cause an rcd to trip when a heavy load is drawn on a different circuit. Can you work out why or how?

Thanks Andy, that's really helpful too!  I'll let you know if I managed to fuse the lot. 

Cheers!

 

[Andy™]

i gave you a clue to that one. think of the voltages on the neutral compared to earth whilst a high load is applied

 

[not_shocked_yet]

Sorry, its too late and my brain is already hurting on other stuff - missus not helping!

I will have an answer for you but I need some time to think and research.

Not always.....

The regs does say that insulation resistance tests should be conducted with the protective conductor connected to the earthing arrangement (reg 612.3.1) but if I am testing an individual circuit on an individual MCB and I have removed the neutral from the neutral bar surely I test against the earth removed from the earthing bar as well?  Or does it not make any difference?

 

[Andy™]

and what if the circuit in question has a fault between a live conductor & an earthed part (i.e damaged sheath up against a copper pipe)? you IR wont show that if you only test to a disconencted earth on the outgoing cable

 

[not_shocked_yet]

Hi Andy,

Good point.  But how do you isolate the circuit with the earth fault then if the fault is on a different circuit to the one you are testing - surely it could be on any of the circuits as all the earth's are connected?

 

[Andy™]

its the live conductors that are at fault to earth. it doesnt make any difference if you test it with or without the earth connected, but once you have identified the circuit, testing with earth disconnected will tell you if its a fault to earth within the cable to or something outside the cable

 

[ProDave]

Pro Dave, can you expand a bit ion yesterday's question, not sure if the answer:

But that's not the end of it.

A N-E fault can cause an rcd to trip when a heavy load is drawn on a different circuit. Can you work out why or how?

Thanks Andy, that's really helpful too!  I'll let you know if I managed to fuse the lot. 

Cheers!

An RCD detects imballance between L and N.

So a correct circuit, all the current leaving L will pass through the load and back through N so there will be no imballance.

Remember N is joined to E at the substation.

Now introduce a N-E fault.

SOME of the current will now return to the substation via E and literally through the earth.

So now there will be a difference in the current flowing out of L and the current returning via N, so the RCD will trip.

 

[not_shocked_yet]

Ahh, thanks Andy I get it!  I appreciate your input, many thanks.

 

[apprentice87]

As Andy said, testing with the CPC disconnected will not tell you much. I do it on things like SWA before i install it, [would not be nice to install loads of that stuff and then find it was faulty] and i do it as part of the dead tests i do, mainly to check it is not all going to explode when i power it up [in case i have made an error in the wiring, better safe than sorry]

BUT, i always test again with the CPC connected, as then, you will detect faults between your conductors and earth as Andy said. How else you going to do this unless the CPC is connected. Be like trying to measure the voltage of a 9v battery, but by only touching the one terminal... Not going to work!!

john...

 

[apprentice87]

Ok, who descoobed me and why??

john...

 

[Andy™]

no idea, but FTFY