e.g. Screwfix RCBOs order code 99608

http://www.screwfix.com/p/legrand-20...!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.