Hi John,
In a case like this both or either one of the RCDs could trip with an earth leakage fault. It will depend on the exact tripping current an tripping time. Both RCDs will probably be rated at 30 mA trip current. I can't remember the trip times. I do not see any point in having a second RCD if the circuit is already protected by one. He will know the house RCD has tripped if the item he is using stops. (If it stops until he goes to the consumer unit and checks if the RCD or the MCB has tripped he will not know if the loss of power is due to an earth fault or an over current fault.)

Les.

Surely it isn't going to matter if you have a second ground. Ground is ground is ground. The neutral is grounded at the substation, so ANY path from live to ground outside of the neutral should trip the RCD.

For this scheme to work, you would need to either run an unprotected live to the outside RCD or possibly use an isolation transformer and then ground the neutral and connect up the socket via an RCD.

Unless you have a dedicated garage consumer unit with its own master RCD or use individual MCBOs for each circuit there isn't a neat solution. The last thing you want is to trip the whole garage out, machines, lights and all when you are in the middle of a potentially dangerous operation.

I'm not convinced that a series of RCDs with reducing trip currents is the right solution either.

That's how it seems to me at any rate.

Murray

Hi Frances,
I think your understanding of how an RCD works is wrong. It does not measure current in the earth conductor. If it did that it would only trip with leakage to the EARTH CONDUCTOR. It would not trip if there was leakage from live to some other item that was at ground potential. Such as a water pipe or through you to damp soil.
The RCD works by comparing the current in the live and neutral conductors. If they are not the same then there must be current flowing to ground. Which is anything grounded by any route. So in this case the example of current flowing through to damp soil will trip the RCD. I agree with all of the comments made by Murray.
John, Just putting a neon indicator in the garage fed from the ring main would solve his problem. (Or a relay which sounds a buzzer if the power to its coil is removed.

Les.

I think I know how a RCB works – on difference between Live and return neutral currents – my original comment may have been too simple but for standard UK houses with short cable runs and earth as supplied by Elec Company the capacitive leakage is probably not serious and trips would indicate a side path between live or neutral supply leads to earth (often caused by poor insulation) – possibly an immersed pump in one of the tanks has poor insulation?

I was assuming garage is very close to the house and that earth is that supplied by the Elec company – if you are in a rural (long supply) and/or long spur out to garage then you need significantly different earthing arrangements than if you are in town or suburb housing estate – in that case the installer would have provided the necessary separation between house + garage supplies

Edited By Frances IoM on 16/09/2015 21:11:59

Woody, Congratulations on finding a sparkie who knows his stuff, they are rare nowadays!

Phil

Primary point. Anything connected should not be tripping the breakers at all. It's a fault and needs to be fixed.

Secondary point. Either fit a separate consumer unit in the garage with a feed from an MCB on the main board in the house fed from the supply side of the house RCCB or just rely on the protection on the main board in the house and don't use a plug in.

Plug in RCD's were a good idea when the majority of domestic installations were wire fuse and earth leakage protected only. With a lawn mower the biggest risk is a cut cable which does not always generate an earth fault and on the old installations could give you a belt without tripping the earth leakage trip.

Just as a side note, current spikes caused by inducive loads switching on and off and non sinusoidal waveforms caused by chopper control of current can cause protection devices to misbehave.

MCB = Miniture Circuit Breaker (electronic fuse)

RCCB= Residual current Circuit Breaker (detects faults by imbalance between the live and neutral conductors)

RCD = Residual current device

Hello Les,

I can see the debate continuing for some time, I hope my second posting made my installation a little clearer.

Just to clarify any confusion I may have caused.

The arrangement here before change was completely conventional, the workshop supply cable is protected from excess current with a miniature 32 A circuit breaker; the whole board of the consumer unit is protected from earth leakage fault by a 30 mA residual current device. Earthing at the consumer unit is referenced back to a standard earth rod close by

The new arrangement differs only in the provision of a second 30mA residual current device supplying the workshop and garden circuit alone; that is now referenced back to the second earth rod separated from the other by about 70 feet. There is now no direct connection back to the earth as seen at the consumer unit.

When this new arrangement was tested with direct phase to earth faults in the workshop, we also took measurements of the tripping current for both live to earth and neutral to earth. I don't remember the individual values now but they were in the range of 28mA and 31mA, but as I said in posting 2, the house supply continued to work uninterupted on both these tests—exactly what I had been told and expected it to do.

I think Alex Collins has added useful insight to all this with his long cable runs and the induced votlages that can build up accordingly, earthing to the planets surface will have variability from water content alone and cannot therefore be at equal potential at all points. Each point of earthing is still however equally valid as a reference point from which to measure current loss. As Phil Whitley has observed, I think I am fortunate to have a sparkie who knows his stuff.

Brian

Edited By Brian Wood on 17/09/2015 09:45:31

Hello Les,

I don't know who David is but I suspect your comments were aimed at me!

I thought all was clear, but on the point you raise, The second RCD, which now protects the service to the workshop and garden area, is interposed into the supply from the house consumer unit. So, technically, the phase connecrions through it are from the RCD ptotected supply and the 32 Amp circuit breaker on the house board.

The earth reference back to the house RCD is disconnected and that reference now comes from the second earthing rod.

You will I hope remember that the requirement for this arrangement was to maintain the house services intact in the event of an earth fault in the outer circuits. It does what it said on the box and rather well I think.

Does that make things clear now?

Brian

That's what we had in Cambridge, as our house had a TT earthing systems due to the overhead power lines which came in without a ground. In a TT system, a local ground is essential for the RCD to work into.

I had a garage consumer unit with separate RCDs for the lighting and power so that earth leakage trips due to my machines didn't plunge me into darkness (alternatively, individual RCBOs would have done the trick). I think the main incomer RCD was actually something odd like 130mA but it came from The Ark. The local ground was provided by the water pipes but luckily the water and gas incomers were all plastic, rendering the main RCD rather useless. Let's just say I later fitted a ground rod after some interesting experiences.

Sort of questionable whether there is any point having the 100mA RCD in a normal TN-S system where you have separate local RCDs for the house and garage. The only obvious benefit is that the spur to the garage has RCD protection, in which case you may as well just use the 100mA trip for the garage spur alone.

Certainly, unless you insert an isolation transformer in the spur, fitting an local ground spike for a local RCD is pointless and can't be counted on to operate as hoped for. Sounds as if that electrician has succeeded in selling The Emperor's New Clothes.

Modern consumer units now have multiple (usually 2) RCDs to prevent the whole house losing power if there is a ground fault. And an additional RCD isn't expensive if you shop somewhere like Screwfix.

Murray

Hi Brian,
I don't understand how it can work connected in the way you describe.

Les.