MCB RCD Problem

[Flying Sparks]

I could do with some help / guidance.
Last week I fitted a new CU into an old cottage. The house was rewired approx 30 years ago.
The Ring Final Circuits r2(2.01) was much higher than rn(0.38) & r1(0.43). 6mm & cpc 2.5mm
When I did the Continuity test I found the R1 + R2 was 2.27.
The Insulation Resistance was lower on this system ( 255 M ohm) - other systems normal.
When I plug anything into the the Ring Main it trips both the RCDs and the MCB for the Ring Main only.
Any suggestions?
Thanks

 

[malcolmsanford]

You had a 6mm RFC !!!! ......................think you may find that this was some old 7/029 which is about 3mm in new money.

That may have been a 1.00 mm CPC you had there.

255M Ohms will not trip the RCD, that is easily OK.

Well if it was just the RCD tripping I would say that you either mixed up the RCD sides, ie neutral in the opposite bar from the Lives.

As you got ring continuity by the sounds of things it's not an issue with polarity, ie Neutral/Earth swopped somewhere.

So why would the MCB trip as well...................hmmmmm you might have to start breaking this ring down

 

[Richard Burns]

As Malcolm says: you have a RFC wired in 6mm2 T&E?


Tripping both the RCDs indicate that there is a neutral swapped between circuits across the RCDs.
The tripping of the MCB as well is very odd since the RCDs would trip first and prevent the short circuit (L to N) that must be present on the RFC from tripping the MCB. Is the ring circuit not RCD protected?


I would say that you have mixed up two circuits when rewiring the new CU.

 

[Engineer54]

.think you may find that this was some old 7/029 which is about 3mm in new money.

Could even be 7/O36, often used in the old days for ring mains and always used for the single socket in the kitchen...lol!!!

 

[malcolmsanford]

Could even be 7/O36, often used in the old days for ring mains and always used for the single socket in the kitchen...lol!!!

Blimey Col that is nearly 5mm that baby, must have had a few 3kw lecy fires hammering away on your RF where you were mate ............

I remember the old 15amp sockets in 7/036, for as you said in kitchens. Many a 6kw cooker with the cables inserted into the socket with a few matchsticks to give it mechanical strength .................... these lads today eh would have a fit.

Best I saw was a single 15amp socket in a kitchen the cooker pushed into a cubed adapter and then 4/5 more adapters into that for fridge, kettle, washing machine etc, all held up by a bit of 2X1 cut exactly wedge from the floor to the adapters to stop them drooping , and not a scorch mark in sight

 

[plugsandsparks]

I know this may be a long shot but "tripping the MCB when a load is applied"

You may have a near short on the wiring somewhere which completes when a small load is applied, this may account for the lower resistance as a hint to something not quite right. When you say old cottage, is this nice and damp with one of those electric damp proof courses?

Other wise as others have said, time to start splitting the ring as soon as you have confirmed neutral is in right side of CU

 

[Engineer54]

The installs where i've seen them used, were on tied cottages built and owned by the local factories, quite a few in fact in and around Purfleet where all the old cement factories used to be located. That Kitchen socket was mainly used to supply the old washing boilers (can't remember the name for them). The electric kettles also used that socket or be plugged into the cooker unit outlet. As far as i remember the old cooker cable was 7/044, certainly was in my parents house... May all have been installed by the old industrial electricians from those companies, they always like to go bigger rather than smaller...lol!!

Most of these cottages have well and truely gone now, making way for the Lakeside developments. I don't recognise the place anymore...lol!!!

 

[Flying Sparks]

The N is on the right side of the CU. Checked, Checked & Checked again.
Old cottage, damp. Boxes are covered in rust. Very damp house.
A microwave was connected - no other load.
I don't know where all the cables are so it's not easy.
There has been added to the system several times.

Thanks for all the replies.

 

[plugsandsparks]

Well hopefully there are not too many sockets - might be worth going back to IR , make sure nothing is in and hit it with 500V again. Try it LN , LE and NE mking sure its all disconnected at the board. I know this is going to be a long job but start taking off the socket plates , you can get some eletrolysis corrosion in them sometimes. I would hope the fault is immediatly obvious when you start looking behind the socket outlets, sooner you than me though

 

[Flying Sparks]

I'll check again tomorrow to make 100% sure I haven't mixed them up, but I'll check again tomorrow. The ring circuit is RCD protected but it keeps tripping both RCDs & MCB for the ring circuit.

 

[Brightspark2]

I had a problem like this last week, RCD would trip when power switched on but everything tested out ok. Had to split the circuit up into smaller parts and eventually found the problem to be a joint hidden in the wall so had to cut the cable's in the void in joint it through and they lost the socket.

So as said time to start splitting the ring up to find the problem...

 

[Engineer54]

I'll check again tomorrow to make 100% sure I haven't mixed them up, but I'll check again tomorrow. The ring circuit is RCD protected but it keeps tripping both RCDs & MCB for the ring circuit.

As has been advised many times on these forums, it's always better to undertake IR tests etc, ...Before you start work on changing CU's, especially checking for the dreaded borrowed neutral on lighting circuits. If everything was ok before your work commenced, then it should be fine after your work is finished. If it's not, then at least you know it's something you have done, which greatly limits your fault search area to the CU in most circumstances...

 

[Flying Sparks]

You had a 6mm RFC !!!! ......................think you may find that this was some old 7/029 which is about 3mm in new money.

That may have been a 1.00 mm CPC you had there.

255M Ohms will not trip the RCD, that is easily OK.

Well if it was just the RCD tripping I would say that you either mixed up the RCD sides, ie neutral in the opposite bar from the Lives.

As you got ring continuity by the sounds of things it's not an issue with polarity, ie Neutral/Earth swopped somewhere.

So why would the MCB trip as well...................hmmmmm you might have to start breaking this ring down

I Have sorted the problem. Thanks to malcolmsanford.
When I looked at the system again I found the N connected to the other RCD.

Thanks for all the replies I received.

 

[Deleted member 9648]

The N is on the right side of the CU. Checked, Checked & Checked again.
Old cottage, damp. Boxes are covered in rust. Very damp house.
A microwave was connected - no other load.
I don't know where all the cables are so it's not easy.
There has been added to the system several times.

Thanks for all the replies.

I'll check again tomorrow to make 100% sure I haven't mixed them up, but I'll check again tomorrow. The ring circuit is RCD protected but it keeps tripping both RCDs & MCB for the ring circuit.

I Have sorted the problem. Thanks to malcolmsanford.
When I looked at the system again I found the N connected to the other RCD.

Thanks for all the replies I received.

Lesson learnt there mate.
But a wrong N connection would only affect the RCD.....what was the cause of the MCB tripping as well????

 

[plugsandsparks]

Lesson learnt there mate.
But a wrong N connection would only affect the RCD.....what was the cause of the MCB tripping as well????

Sympathy?