Didn't think was possible

[Pondy]

Ok guys, thought I would share this with you, have a call to a one bedroom flat with rcd tripping . It's the main flat ring which is the one. It's a tns system, incoming supply is pyro. 16th edition board(crabtree), lights and smoke on none rcd side,kitchen, flat, cooker and unknown circuit on rcd side. the unit is in the kitchen. The kitchen has had a new ring and cooker circuit installed I'm guessing at the same time as the board was installed.
Originally the flat had one ring and so they have though crimped in a couple of places where was in the kitchen and these are behind double blank plates. I have inspected these crimps and they are fine.
This is the part I really can't get my head around, even with all circuits off and all neutrals out and even the main neutral coming out of the rcd removed, but all earth's still in place the rcd still trips when trying the flat ring. I will be re booked to fix, the guy is using an extension lead from the kitchen for power in the bedroom and lounge. All items have been unplugged.
Coincidentally I'm not sure whether this could be related, the ze is around 3.5 ohms and the zs on the kitchen ring is around 80 ohms. The r1.r2 and rn is .13 .26 .13 respectively. I done a zs at home and it was fine, new batteries in my fluke tester., so doubt tester at fault. I haven't done any insulation resistance testing yet as time was short. Just baffled by the rcd tripping with no neutral. Your thoughts or suggestions would be most grateful.

 

[uksparks]

3.5 ohms for Ze? That's very high, is this fed straight from the meter from cutout? You will not meet disconnection times on your sockets with that.

 

[Pondy]

Yea I know the max ze for this system is 0.8ohm
. Will be contacting the supplier. I moved the kitchen ring over to the non rcd side temporary and same reading

 

[Plonker 3]

I hope you then put it back on the RCD side once you had done this?

 

[Pondy]

Yes of course. It was only temporary to make sure rcd wasn't giving me the high reading

 

[tony mc]

Ok guys, thought I would share this with you, have a call to a one bedroom flat with rcd tripping . It's the main flat ring which is the one. It's a tns system, incoming supply is pyro. 16th edition board(crabtree), lights and smoke on none rcd side,kitchen, flat, cooker and unknown circuit on rcd side. the unit is in the kitchen. The kitchen has had a new ring and cooker circuit installed I'm guessing at the same time as the board was installed.
Originally the flat had one ring and so they have though crimped in a couple of places where was in the kitchen and these are behind double blank plates. I have inspected these crimps and they are fine.
This is the part I really can't get my head around, even with all circuits off and all neutrals out and even the main neutral coming out of the rcd removed, but all earth's still in place the rcd still trips when trying the flat ring. I will be re booked to fix, the guy is using an extension lead from the kitchen for power in the bedroom and lounge. All items have been unplugged.
Coincidentally I'm not sure whether this could be related, the ze is around 3.5 ohms and the zs on the kitchen ring is around 80 ohms. The r1.r2 and rn is .13 .26 .13 respectively. I done a zs at home and it was fine, new batteries in my fluke tester., so doubt tester at fault. I haven't done any insulation resistance testing yet as time was short. Just baffled by the rcd tripping with no neutral. Your thoughts or suggestions would be most grateful.

I can't get my head around what you are saying re neutrals remove and RCD still trips when trying flat ring can you explain how you are testing !!

Are you connecting the ring only both Line & Neutral of the RFC ?

Have you tested the RCD on its own no loads to see if its faulty?

IMO a IR test of the FRC must be high on the list of testing especially when you have you test gear out !

I would not rule out any of the other ccts either!

The maths re you ZE and results just don't add up either with that 80 ohms figure !!!

 

[Engineer54]

My question is, where are you taking your Ze reading from? The flats CU or at the main incoming supply to the building??

If at the flats CU, that's known as Zdb it's basically a Zs value, and you have a problem on the sub-main supply cable between the main incoming supply and the flat.

If it's at the main incoming supply to the building then you have a ''REAL'' problem, that is not only affecting the flat you're working in but all the other flats as well. If that's the case, cease all work in this flat and call the DNO emergency hotline, which you should have done immediately you found the Ze fault in the first place!!!...

 

[Pondy]

E54, that's a very good point and one which I overlooked, annoyed at myself for not thinking about that, I was there on an emergency basis and will be going back this week. That's why I wondered whether my tester was faulty as I know it's not possible, either that or I'm doing something wrong. Just to clarify the other bit, with all neutrals out of the board and the neutral taken out the outgoing side of the rcd it still tripped on the flat ring

 

[Pondy]

E54, the first thing I will check is the ze at origin. I'll let you know

 

[Octopus]

Are you saying that you removed all the load neutrals and the RCD trips? Removing neutrals isn't likely to prove anything really IMHO

 

[Richard Burns]

If you have a fault from line to earth then this will trip the RCD, it would not matter if the neutrals were in place or not.
You could also have a neutral to earth fault that is then tripping the RCD as the current through the load to the neutral to earth fault will still be enough to trip the RCD.
Since the RCD trips all the time then you need to eliminate anything plugged into the circuit and then test the circuit, there should be a clear and obvious low IR result somewhere.

 

[Pondy]

Ok, I swung by on way home and sure enough the ze at the origin is 3.5 ohms so all four flats are affected. Have emailed my office saying as a matter of urgency we need to contact the supplier.
I always thought that a rcd looked for an imbalance between neutral and earth,and a mcb took care of overcurrent and short circuit and earth fault, hence being stumped after removing all neutrals from the board and even removing the outgoing neutral from the rcd and the rcd still tripping

 

[davesparks]

An RCD does not look for an imbalance between neutral and earth, it doesn't have an earth connection, and you'd have serious issues if there was a balanced current across neutral and earth!

 

[Pondy]

Sorry, I wrote that very quickly and without reading before sending, I meant it measures the return via the neutral which should be equal to the line conductor, any leakage via the earth will trip the rcd.

 

[SirKit Breaker]

You say you havent done the IR testing yet. Instead of clutching at straws, start at the beginning. Do your continuity tests, IR and see what these highlight, it may put you straight to the answer. Generally, if you do things in the correct sequence it may appear to a be bit time consuming, but overall it will generally be quicker than starting in the middle and best guessing, not to mention forgetting what you have already tested. You may find a high resistance that might only be a loose connection, but enough to upset the RCD and trip it out.

Cheers................Howard

 

[guest119]

And don't assume there is always just one fault,there may be more or an accumulation.

Boydy

 

[PEG]

Ok, I swung by on way home and sure enough the ze at the origin is 3.5 ohms so all four flats are affected. Have emailed my office saying as a matter of urgency we need to contact the supplier.
I always thought that a rcd looked for an imbalance between neutral and earth,and a mcb took care of overcurrent and short circuit and earth fault, hence being stumped after removing all neutrals from the board and even removing the outgoing neutral from the rcd and the rcd still tripping

Hi Pondy,an RCD does monitor and operate due to an imbalance,but between line and neutral,this may be shootin' up THE earth,or AN earth,but it determines "some",maybe 30 ma's worth is awol,and operates accordingly. The advice concerning the order of tests is sound,and although on occasion skipping some can still bring a quick result,in general it negates back-tracking. Happy hunting.