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[florence44]

I am thinking of installing a new Cons Unit, and fitting it with RCBO's,but I am worried that they will not 'see' Neutral Earth Faults. Can anyone advise me on will they detect Neutral Earth Faults, or just Phase to Earth faults, and Overcurrent?-thanks.

[ericmark]

The RCBO will detect exactly the same as an RCD. However some RCBO's are only single pole switching although they still detect both poles. The single pole switching devices can only be used with a TN system. With a TT system it needs a twin pole device. Or it needs a RCD which is twin pole switching as the main isolator (could be 100ma though).

As to detecting the RCD detects the imbalance between the two live conductors (Line and Neutral) also for where volt drop could cause them to fail they may also detect under voltage (This type is called active).

Since the neutral is closer to earth voltage it will not detect an earth neutral fault as easy as an earth line fault. This goes for all RCD's. The earth neutral fault becomes more apparent as the voltage differential increases and this happens as more current is drawn. Hence why plugging in the kettle can trip a RCD when the fault is in a standard lamp.

The problem with RCD protection not switching the neutral is a earth neutral fault is not removed when the RCD trips and can result in other RCD's also tripping as a result. However in general having many RCD's means you don't get the build up of leakage from many devices tripping the RCD when it is not required but it still will trip the RCD when needed.

So I would where possible always use RCBO's. You can get double pole RBCO's but the distribution boards that go with them are not so easy to get and often are not of the type tested type called consumer units. Abroad they are more common but in this country the TT earthing system is less common than abroad.

[florence44]

[quote="ericmark"]The RCBO will detect exactly the same as an RCD. However some RCBO's are only single pole switching although they still detect both poles. The single pole switching devices can only be used with a TN system. With a TT system it needs a twin pole device. Or it needs a RCD which is twin pole switching as the main isolator (could be 100ma though).

As to detecting the RCD detects the imbalance between the two live conductors (Line and Neutral) also for where volt drop could cause them to fail they may also detect under voltage (This type is called active).

Since the neutral is closer to earth voltage it will not detect an earth neutral fault as easy as an earth line fault. This goes for all RCD's. The earth neutral fault becomes more apparent as the voltage differential increases and this happens as more current is drawn. Hence why plugging in the kettle can trip a RCD when the fault is in a standard lamp.

The problem with RCD protection not switching the neutral is a earth neutral fault is not removed when the RCD trips and can result in other RCD's also tripping as a result. However in general having many RCD's means you don't get the build up of leakage from many devices tripping the RCD when it is not required but it still will trip the RCD when needed.

So I would where possible always use RCBO's. You can get double pole RBCO's but the distribution boards that go with them are not so easy to get and often are not of the type tested type called consumer units. Abroad they are more common but in this country the TT earthing system is less common than abroad.[/quote]
Thank you for replying so quickly, and explaining it to me, much appreciated.

[russell fields]

neutral and earth are both at zero volts potential, therefore a neutral to earth fault is of no danger.
in TN systems the neutral is physically connected to earth, in TN-CS systems this connection is in the distributors cut-out
there is no requirement to test an RCD or RCBO under neutral to earth fault conditions.