Academic questions regarding rcds.

[frank rabbets]

I've just introduced myself and have two questions regarding rcds.

I understand these monitor "ingoing and outgoing" power and if there is a sufficient imbalance the system assumes there is loss to earth and trips the circuit for safety.

My questions are:-

1. Is there a natural leakage to earth in cables and equipment in which case a very large house would need a different RCD than a small house ? I.E. is the RCD "tuned" or selected for its' environment?

2. If there is an RCD in a house and a cable from the house is run to a detached garage is the garage area protected by the RCD in the house ? I assume so.

Thanks in advance.

 

[telectrix]

1. Is there a natural leakage to earth in cables and equipment in which case a very large house would need a different RCD than a small house ? I.E. is the RCD "tuned" or selected for its' environment? if there is sufficient leakage in a installation to trip the RCD, then it's a fault/s that need rectifying.

2. If there is an RCD in a house and a cable from the house is run to a detached garage is the garage area protected by the RCD in the house ? I assume so.Not always. usually the rCD would be in the outbuilding so that a fault there would trip locally and not in the house, possibly taking out part of the house installation with it.

 

[Midwest]

1. Some appliances do have functional earth leakage, e.g. computers etc. Some also cause problems as they get older, e.g. dishwashers as the heating elements break down with age.

Suitable design of circuits and use of separate RCBO's would prevent this scenario arising in most cases.

Why do you ask OP?

 

[frank rabbets]

Purely academic question. I should not have referred to appliances just the leakage in wiring only of a large house compared with a small house. Thanks for replies I wonder if you would consider the corrected question.

I don't have rcds.

I understand now why you would have an rcd in the garage itself thanks.

 

[telectrix]

the RCD in the garage would not be acceptable if the feed to the garage itself required RCD protection, e.g. if it were a TT earthing system, using RCD for fault protection.

 

[Octopus]

The size of the house is irrelevant - unless it has multiple kitchens and the associated appliances....

RBCO's are the way to go.........

 

[Rpa07]

Great user name - I once knew a Frank Rabbit, always got to the point (of the carrot) quickly.
Oh Mondays!

 

[Taylortwocities]

Is there a natural leakage to earth in cables and equipment in which case a very large house would need a different RCD than a

Cables and wiring do have a very small degree of insulation resistance - it is a natural effect of having conductors close to each other.
But, even in the largest installation, one would expect to see an insulation resistance test returning a value in excess of 200Meg ohms.
BS7671 allows a IR value of 1Mohm, but I would be looking for a fault if i saw that value on a new installation.



older installations do degrade over time, but IR values tend to decrease more as a result of damp, or dust and dirt on terminaations, rather than a breakdown in conductor insulation.

You may find this of some interest
The RCD Handbook - Beama

 

[davesparks]

1. Is there a natural leakage to earth in cables and equipment in which case a very large house would need a different RCD than a small house ? I.E. is the RCD "tuned" or selected for its' environment?

2. If there is an RCD in a house and a cable from the house is run to a detached garage is the garage area protected by the RCD in the house ? I assume so.

1. There can be all sorts of small leakages in an installation.
RCD tripping current is selected according to the type of protection required, eg 30mA for protection against fatal electric shock.
Instead of 'tuning' the RCD to the installation as you put it you instead split the installation up across multiple RCDs. Ideally every circuit would have its own RCD (in the form of an rcbo usually)

2. Yes if the circuit to the garage is connected to the RCD in the house then everything in the garage is protected by that RCD. However it is better to separate the garage onto its own RCD (located in the garage if the installation method allows it)

 

[frank rabbets]

Thanks for the very clear answer Dave. I never got it that you could split a large house to " spread the load", obvious really. Also this helps you when you get a trip as part of the house is still on. As people say the obvious answer is to have a trip on each circuit. This way a fault can be immediately isolated to one circuit and all the rest of the circuits are still usable. Having only 1 RCD is a bit like having one fuse for the whole house lol.

I read the whole of the RCD handbook thanks but did not understand the maths.

I was told by an electrician that if there was RCD in a house this would not protect the garage even though, rightly or wrongly, this was connected through the house RCD. I could never understand why, hence the question.

Thanks for all the contributions.

 

[telectrix]

I was told by an electrician that if there was RCD in a house this would not protect the garage even though, rightly or wrongly, this was connected through the house RCD. I could never understand why, hence the question.

you were told wrongly. the RCD in the house , supplying the garage, protects both the garage and the feed to it ( if the feed needs protecting, which would be the case with a TT earthing system, or if feed cable was buried in wall, < 50mm deep , e.g. a T/E feed to the outside of the house ). It's not always a good idea though, as it's better for a fault in the garage to trip locall to the garage, rather than in the house.

 

[davesparks]

I was told by an electrician that if there was RCD in a house this would not protect the garage even though, rightly or wrongly, this was connected through the house RCD. I could never understand why, hence the question.

Unfortunately there are a number of electricians out there who don't understand these things which should be the basics for them.
There are people who are falsely under the belief that if a separate earthing system is set up in the shed (which is sometimes necessary but I won't go into details and confuse the issue) then the RCD in the house won't trip on a fault in the shed, this is untrue as the RCD doesn't really care about the earth, it only monitors live and neutral.

 

[Octopus]

Some so called sparks think that making an out building TT will stop the RCD at "source" tripping......................

The best set up is RCBO's imho...