Just curious.

[conny]

If you have a supply to a shed/garage, and the circuit is 30mA RCD protected, there is no need for another RCD in the shed/garage CU.
However, if a fault happens and causes the RCD in the main CU to trip, this can become a nuisance having to go indoors and reset it.
Would the answer be to, isolate the earth from the main CU in the house, and instead, run an earth from the shed/garage RCD protected CU, to an earth rod in the ground outside? Would this prevent the main RCD tripping?
This is not a current problem. Just one of those idle thoughts that pop into your head when you are relaxing.

 

[DetlefSchmitz]

If the current between the two live wires in the garage differs, then it will differ in both the garage rcd and also in the house rcd. How else could it be?

 

[Murdochcat]

Isolating the earth WILL NOT prevent a rcd tripping …. Not having an earth would - but don’t do that as it would be rather dangerous under fault conditions

 

[EFLImpudence]

SWA is used for the cable run from house to shed/garage so that the cable does not have to be 'protected' by an RCD in the house.

 

[EFLImpudence]

Technicaly it still does have to be protected....

Not by an RCD.

 

[oldbutnotdead]

If it is SWA all the way then best solution is Henley blocks and feed the garage (via suitable fuse/circuit breaker) before the CU. Then RCD and baby CU if appropriate in the garage.

 

[ericmark]

The RCD looks for an imbalance, it does not with today's versions look for current to earth, so having two lighting circuits and selecting the wrong neutral will trip the RCD even with no current to earth.

The old ELCB-v did measure earth voltage, if it exceeded 50 volt it would trip, however it was too easy for bonding to cause the unit to become inactive, so they stopped being used. There was a list of cables where RCD protection was not required, BS 5467,BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436, what I thought was odd is SY cable is not in the list, maybe it is now.

Where a shed supply comes from a FCU there is not really a way to have the RCD in the shed, it was common to run twin and earth inside the house, and only use SWA outside, that also often stops one being able to have the RCD in shed rather than main house.

 

[Taylortwocities]

SWA is used for the cable run from house to shed/garage

The OP hasn't said that SWA is being used for this circuit.

 

[EFLImpudence]

He hasn't said anything.

He is just asking a question.

If you have a supply to a shed/garage, and the circuit is 30mA RCD protected, there is no need for another RCD in the shed/garage CU.
However, if a fault happens and causes the RCD in the main CU to trip, this can become a nuisance having to go indoors and reset it.
Would the answer be to, isolate the earth from the main CU in the house, and instead, run an earth from the shed/garage RCD protected CU, to an earth rod in the ground outside? Would this prevent the main RCD tripping?
This is not a current problem. Just one of those idle thoughts that pop into your head when you are relaxing.

 

[Taylortwocities]

He hasn't said anything.

He is just asking a question.

Ah OK. Then your post should have said
IF SWA is used for the cable run from house to shed/garage THEN the cable does not have to be 'protected' by an RCD in the house.

Actually, that is assuming the SWA goes all the way to the CU. If unprotected cable is used inside the house, then the circuit may may still need RCD protection, depending on the cable route.

 

[EFLImpudence]

No, it shouldn't.

Conny is asking what happens with an RCD in the house and an RCD in the shed and I said:

SWA is used for the cable run from house to shed/garage so that the cable does not have to be 'protected' by an RCD in the house.

...and of course you have to add that if something else is done then what I said does not apply.

Duh!

 

[conny]

To try and clear any confusion.
As I said at the start, this is just one of those idle questions that sometimes pop into my head when I'm sitting down relaxing. It is not a current situation, nor one I am envisaging at some future date.
To clarify the talk about SWA, I always work on the basis that anything outside the house has to be in suitable SWA, be it direct from the house CU to another CU in shed/garage, or from an outdoor, waterproof termination box at the house to the same at the shed/garage, (or any other combination of the two. i.e. House CU to shed box or house box to shed CU etc).
Hope it clears that little point up.

 

[EFLImpudence]

To clarify the talk about SWA, I always work on the basis that anything outside the house has to be in suitable SWA,

It doesn't.

As well as the over-hyped so-called armour protection, it is used to avoid having to have an RCD at the house end which would be required with other cable and some installation methods leading to the disadvantages you mentioned in the original question.