Will CU trip on a lighting circuit for a fault if no earth?

[p_a_johnson]

For a lighting circuit without an earth (i.e. - without rewire), will the circuit breaker (is it an RCB or an MCB or something - you'll have to excuse my lack of knowledge!) on the consumer unit trip out if a person came into contact with a live/ switched live on the lighting circuit or would it allow supply oblivious to the fact that he was getting fried?

 

[breezer]

if you have an rcd then it should trip, if not the person will be long dead. an mcb is not the same as an rcd, an mcb is to protect from overload an rcd isnt, for more about an rcd see this

but if ther is no earth then it also implies the wiring is old, which means its time for a re wire

 

[Big_Spark]

but if ther is no earth then it also implies the wiring is old, which means its time for a re wire

The installation should be rewired regardless of age if there is no Earth .

 

[plugwash]

the main function of an earth is to provide protection against indirect contact

so if all of your lights and switches are non metal you should be safe but it still doesn't comply with regs (btw does anyone know why you wan'te allowed non-earhte lighting cuircuits if you use fittings without exposed conductive parts)

borrowing an earth from a socket for a metal fitting will make the fitting safe for now but does not comply with the regs becuase of the possiblility of it getting disconnected and the supply conductors left in place

i would advise rewireing the lights and anything else you know is dodgy in the modern way then get a sparky in to do a full periodical and make sure the main earth is ok

 

[p_a_johnson]

cheers plugwash

what is entailed in a "full periodical"?

does he just come armed with a flash multimeter and check all the circuits or something?

 

[plugwash]

yeah he will come round with his test kit and test everything
then he will tell you what remidial work is needed if any before the install can be certified as ok

most sparkys i think would be rather annoyed of you hire them for a periodical then don't get them to do the remidial work which is why i suggest replaceing obviously dodgy stuff first

 

[Big_Spark]

Plugwash, without wishing to put your nose out joint, but your understanding of the use of an Earth is inaccurate, to a degree.

Whilst it is true to say that the use of earth does provide protection against indirect contact, it also does many other rather useful things too, like:

Providing a path for the creation of a short-circuit when cables are damaged in a way which does not completley break the cable. This is the reason why the Earth core in T&E has no sheath on it. Should the cable be damaged, outer sheath and one core damaged, there is a reasonable chance of connection with the naked earth wire, this will create a short circuit path and thus result in the operation of the protective device.

Ensuring that the potential difference between the Neutral conductors of an installation and the fundermental mass of earth is reduced to the minimum level, ideally 0V but usually slightly above 0V. This is important as it prevents a person from being shocked by the neutral conductor. There are certain situations where a neutral shock is possible, but the presence of a cpc is irrelevent to this.

The cpc also ensure that all exposed conductive parts, and all extraneous conductive parts are maintained at the same potential difference as the fundermental mass of earth, thus reducing the overall risk of either form of indirect contact.

Without an earth in an installation, there would be a high probability of recieving a shock that would not operate a protective device, and thus increase the chances of a fatal shock.

 

[plugwash]

Providing a path for the creation of a short-circuit when cables are damaged in a way which does not completley break the cable. This is the reason why the Earth core in T&E has no sheath on it. Should the cable be damaged, outer sheath and one core damaged, there is a reasonable chance of connection with the naked earth wire, this will create a short circuit path and thus result in the operation of the protective device.

agreed but then flex has a seperately insulated earth as do singles in plastic conduit

Ensuring that the potential difference between the Neutral conductors of an installation and the fundermental mass of earth is reduced to the minimum level, ideally 0V but usually slightly above 0V. This is important as it prevents a person from being shocked by the neutral conductor. There are certain situations where a neutral shock is possible, but the presence of a cpc is irrelevent to this.

i agree but this about earthing at the transformer in the distribution system nothing to do with household envoronments actually a transformer with neither end strongly connected to earth is in some ways safer the problem is it's harder to provide supplementry shock portection in such a system. rcds are useless on such a system. however with the right breaker systems theese are the safest way going hence why they are used in hospitals.

The cpc also ensure that all exposed conductive parts, and all extraneous conductive parts are maintained at the same potential difference as the fundermental mass of earth, thus reducing the overall risk of either form of indirect contact.

yup metal fittings need earth for this reason but in an all plastic system this is much less of an issue

Without an earth in an installation, there would be a high probability of recieving a shock that would not operate a protective device, and thus increase the chances of a fatal shock.

i agree but then that is what most older houses in mainland europe are like.....

seriously the lights should be rewired but if that is not practical to do immidiatel then earthing metal fittings to the earth terminals of a socket (provided the socket is properly earthed) is a lot better than not earthing them at all