RCD Suggested size problem

[Shaun]

Hi Quick general question re RCD trip currents. Just lately we have been having more (but have always had quite a few in the past) trips of the main RCD in the consumer unit. The current RCD does the whole unit, no split mode here, and is an 80A 30ma one. Is this too low for it to be over sensitive and thus get nuisance trips? Its a large house with abbout 16 RCB's and some feeding outside workshops and barns. It failed whilst we were away which was interwsting as the alarm went flat and called the polide & fire brigade!! Cheers Shaun

[Colin Wilson]

I`ve been out of the game for a while, but if the other RCDs are all 30mA i`d probably go for something a less sensitive for overall protection, ie. 100mA to give you some sort of discrimination.

[Owain]

"Shaun" wrote | Quick general question re RCD trip currents. Just lately we have | been having more (but have always had quite a few in the past) | trips of the main RCD in the consumer unit. The current RCD does | the whole unit, no split mode here, and is an 80A 30ma one. Is this | too low for it to be over sensitive and thus get nuisance trips?

No, 30mA is the correct rating for protection of sockets. NOTE: if your barns contain livestock or quantities of flammable material (eg straw) then the Wiring Regs and your insurance co may have more stringent requirements.

| Its a large house with abbout 16 RCB's

I think you mean MCBs here

| and some feeding outside workshops and barns.

There's your problem. You probably have a high quiescent leakage current on those circuits (it's been raining recently, so dampness will be worse).

A 30mA RCD may trip as low as 15mA, and with 16 circuits you only have an average allowance of

[Smudger]

RCD's that are rated to trip at 30mA will actually trip at less than 30mA (may be as low as about 17mA) and the cumulative leakage current on a large installation may easily rise this high.

It is likely that the rainy weather we have had lately has caused some damp to get into the instalation somewhere (probably in an outbuilding) and the resulting leakage current is causing the increase in your nuisance tripping.

You should first of all thoroughly inspect and test the installation to see if you can find a _particular_ cause of the nuisance tripping.

If you can't find anything in particular, then you should consider replacing your main consumer unit with a split-load model. Then each supply feed (sub-main) to an outbuilding should be on the non-RCD side. Also, your alarms should be on the non-RCD side as well.

Each outbuilding should then have its own small consumer unit which is RCD protected. If you still get nuisance tripping after you have completed this work then you will be able to narrow the problem down to one part of the installation.

HTH

Smudger

[Lurch]

On 15 Aug 2004 03:10:55 -0700, [e-mail] (Shaun) strung together this:

It's a inconvenient setup at best. If you downgrade the RCD to 100A

100mA time delayed if you are on a TT system or a 100A main switch if you are on a PME\TN-C-S system, then do the following....

I assume you mean MCB's. If so, then the circuits that require RCD protection, e.g. socket circuits, the shower circuit, replace the MCB's with RCBO's. RCBO's are devices with a RCD and MCB built into one unit so a fault on the socket circuit will only trip the RCBO for thast circuit and will leave any critical circuits on like CH, alarm, fridge\freezer socket etc..

I'd seperate these from the house supply and put them on individual

30mA RCD's, then if the outbuildings cause a fault the RCD for that building will trip and should leave the house supply on.

Yep, seen that a few times!

[Dave Liquorice]

Firstly does your supply type require a "whole house" RCD?

If not then I suggest it's time to sort out the CU and various feeds. Remove the overall RCD and fit an 30mA RCBOs to each feed to barns/workshops and then a split load consumer unit as per normal (lights, alarm, dedicated fridge/freezer circuit, etc pre-RCD, rings etc post). An RCBO provides both residual current and overload protection in a single device.

If your supply type does require a whole house RCD it's probably still worth doing the above but the whole house RCD becomes a 100mA time delayed one, all the others being 30mA non-delayed, to provide the descrimation.

[John Rumm]

This sounds like it might be a rural location, and hence could have overhead power distribution (i.e. TT system). So the OP may still need a "whole house" RCD (100mA trip though) with more sensitive protection on circuits where required. As you say some segregation of circuits after that would be a good idea anyway.

One problem switching to use RCBOs in an existing CU is that many are two modules wide, hence you run out of space in the box. I did find that TLC Direct do some contactum ones in the special order section that are tall rather than wide. However I would expect these to only fit certain CUs.

[Shaun]

Wow thanks all, what a speedy response :) Thnaks for the extra info, I was wondering why all were put into one consumer unit as all other installations I have seen 9and I was raise don a farm where we had significant electricity requirements0 were based on many smaller CU's providng supplies for various locations. Time to tet a sparky round I think to firstly determine if the leak can be found (I have since the mail disconnected a ring that handles the 2 outside lamposts from both live and neutral) and since no cut, I think the leak is on the neutral side as if it trips and I cut out all the MCB's (thanks for the terminology correction!!) it still trips for a bit. I'll see about splitting out the critical and non critical items to reduce the risk of non-essential circuits cutting out essentail ones. One thing seems common though, I guess the 30ma for the main trip is way too low especially with the demands put on it by soem of the external circuits. Actually I was contemplating putting the barns onto a seperate supply as I run some quite heavy compressors, etc there whcih cause the house lights to flicker and I think its getting voltage drop by the time it arrives, its actually very close to a pole and the local electricity company only wanted about £250 to connect up a new supply from that pole to the workshop (and from there to the barns) so it would then only require a small CU in the workshop to do the rest (fitted and tested as I understand by a qualified electrician). Thnaks gagin for all yuo help and advice, have a good week all :)

"Dave Liquorice" wrote in message news:...

[tony sayer]

In any system where the neutral line is connected to earth at the substation, then RCD protection is essential.

Whether or not you have a local earth or not is irrelevant.....

[Owain]

"Shaun" wrote | Time to tet a sparky round I think to firstly determine if the leak | can be found (I have since the mail disconnected a ring that handles | the 2 outside lamposts from both live and neutral) and since no cut, | I think the leak is on the neutral side as if it trips and I cut | out all the MCB's (thanks for the terminology correction!!) it | still trips for a bit.

Removal of *any* circuit may be enough to take the quiescent leakage down below the trip limit of the RCD. The only way to find out for sure is to remove any electronic devices from the circuits and 'megger' the circuits individually.

| Actually I was contemplating putting the barns onto a seperate | supply as I run some quite heavy compressors, etc there whcih | cause the house lights to flicker and I think its getting voltage | drop by the time it arrives, its actually very close to a pole | and the local electricity company only wanted about £250 to | connect up a new supply from that pole to the workshop (and | from there to the barns) so it would then only require a small | CU in the workshop to do the rest (fitted and tested as I | understand by a qualified electrician).

One useful technique to reduce voltage drop in your barns etc may be to run the submains as a ring circuit (looping from the meter to the individual CUs in each barn and back to the meter). This is NOT a 'standard' 32A socket ring final circuit, but a true ring main at 100A (or whatever your new supply is, 100A single phase is standard), and needs to be designed by an electrician with circuit design qualifications and experience. Also the earthing between different buildings will need to be properly considered.

When the new supply is installed, make sure the interconnect between the house and the outbuildings is disconnected and cut well back to prevent any inadvertent back-feeding of the house from the outbuilding supply, which might well be on a different phase (i.e. 415V instead of 240V difference).

Owain