Annex electricity on a submain

[Taylortwocities]

RCD's aren't tripped by overload only .......... neutral to earth faults too.

Also line to earth faults.
A substantial line-earth fault would trip an MCB, but a smaller earth fault should trip an RCD element if the fault is >30mA nominal.

 

[EFLImpudence]

Confused as @ericmark response suggests that an overload only trips the MCB? Not the RCD too? Is that the case?

That is the case.

There would not be much point having the two devices if they did the same things, would there?

 

[EFLImpudence]

RCD's aren't tripped by overload only .......... neutral to earth faults too.

That doesn't make sense.

The use of "only" implies that they are tripped by overload.

 

[EFLImpudence]

From definitions:

Overload current. An overcurrent occurring in a circuit which is electrically sound.

Fault current. A current resulting from a fault.

Short-circuit current. An overcurrent resulting from a fault of negligible impedance between live conductors
having a difference in potential under normal operating conditions.

Earth leakage current (see Protective conductor current).
Protective conductor current. Electric current appearing in a protective conductor, such as leakage current or
electric current resulting from an insulation fault.

 

[Murdochcat]

That doesn't make sense.

The use of "only" implies that they are tripped by overload.

I was using the OPs terminology

 

[ericmark]

Confused as @ericmark response suggests that an overload only trips the MCB? Not the RCD too? Is that the case?

Yes

However it is operated by current, so the IET/BSi definition of a circuit is satisfied by a RCD, so the rules about why a supply is split into circuits applies to RCD's as well as MCB's and I have never understood how twin RCD's can comply in many cases.

AC leaks due to capacitive and induction linking, so there is always some leakage to earth. An insulation tester uses DC, so until circuits are energised we can only guess what the leakage will be.

So a whole house is wired, and until the supply is connected we have no idea if one or ten RCD's are required, and by that time it's a bit late.

Adding a RCD OK we can measure first, but in the design stage it is a bit of an unknown. Three insulated wires and an earth wire in three and earth and which wire is line and which is neutral will change how much leeks.

I remember doing the calculations, and how far apart the wires need to be in a balanced feeder, and we are careful with 300 ohm ribbon, in mHz range, but twin and earth is so similar but we just use suck it and see.

So safe method is all RCBO, it is so unlikely to cause a problem with so little cable in each circuit.

As to if an electrician puts his clamp meter on and measures over 9 mA what he does I am not sure? Likely crosses his fingers.