Why do some RCBOs have an earth wire and some don't?

[Stephen H]

e.g. Screwfix RCBOs order code 99608

http://www.screwfix.com/p/legrand-20a-30ma-b-curve-rcbo/99607;jsessionId=sfSnRjRbLS3wRBn1DrMmKvg2BVkt3BX5xN9f6vJLGP0J0gh2s3S9!1414933307!1361301851395

which has a neutral blue flying wire and a white flying earth wire for
the neutral and earth CU bus bars respectively and its a legrand RCBO

versus

product code CP BR201 at
http://www.tlc-direct.co.uk/Products/CPBR201.html which is a contactum
RCBO and has a single blue neutral flying lead.

I cannot see a 3rd screw let alone a 2nd flying wire on the contactum
for a fitter to connect an earth wire to the earth bus bar.

As I understand it, an MCB works on the live conductor only and
"measures" the current. It should open once the live current exceeds the
MCB's design trip current. This clearly does not need an earth connection.

A RCD essentially measures the current difference between live and
neutral to the circuit being protected. If the current difference is
greater than 30mA, the RCD opens. I've never seen an RCD in a split load
board have an earth connection either.

An RCBO is a combined MCB and RCD in one unit.

So why do some RCBO's have an earth wire and some don't?

IS one better than the other?

Finally, if an RCBO trips, is there an indicator on it whether its an
earth leakage fault or an overload condition?

Stephen.

[Andrew Gabriel]

In article <EtQUs.31600$s36....@fx22.fr7>,

Stephen H <ilov...@pleasespamme.com> writes:
> e.g. Screwfix RCBOs order code 99608
>
> http://www.screwfix.com/p/legrand-20a-30ma-b-curve-rcbo/99607;jsessionId=sfSnRjRbLS3wRBn1DrMmKvg2BVkt3BX5xN9f6vJLGP0J0gh2s3S9!1414933307!1361301851395
>
> which has a neutral blue flying wire and a white flying earth wire for
> the neutral and earth CU bus bars respectively and its a legrand RCBO
>
> versus
>
> product code CP BR201 at
> http://www.tlc-direct.co.uk/Products/CPBR201.html which is a contactum
> RCBO and has a single blue neutral flying lead.
>
> I cannot see a 3rd screw let alone a 2nd flying wire on the contactum
> for a fitter to connect an earth wire to the earth bus bar.
>
> As I understand it, an MCB works on the live conductor only and
> "measures" the current. It should open once the live current exceeds the
> MCB's design trip current. This clearly does not need an earth connection.

Sometimes an MCB will also provide a ganged switched neutral, but
not normally used in domestic environment.

> A RCD essentially measures the current difference between live and
> neutral to the circuit being protected. If the current difference is
> greater than 30mA, the RCD opens. I've never seen an RCD in a split load
> board have an earth connection either.
>
> An RCBO is a combined MCB and RCD in one unit.
>
> So why do some RCBO's have an earth wire and some don't?
>
> IS one better than the other?

The earth is used on some RCBOs for various different reasons, such as

o tripping if the earth becomes disconnected.
o tripping if live and neutral supply is reversed.
o tripping on loss of neutral connection in supply.
o inducing a tiny neutral-earth voltage to guarantee a trip on
a neutral-earth fault even when no load.
[maybe more I've forgotten]

You would need to find the manufacturer's data sheet to find which
(if any) of these any particular RCBO might offer, but an RCD/RCBO
without an earth connection can't do any of these.

> Finally, if an RCBO trips, is there an indicator on it whether its an
> earth leakage fault or an overload condition?

Usually no, but there have been some RCBOs which indicate by
the toggle being left in the middle for one trip cause, and
all the way off for the other trip cause. Not sure if any
currently available models do this, but it was never common.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]