RCBO v. mini-RCBO

[PeterC]

I've just bought some miniature RCBOs to replace MCBs in my CU - the lighting circuits are on a 100mA RCD as is a spur to a socket in the loft(!).

In a c. 1990 CU there isn't space for full-size RCBOs whereas the minis will fit. What I can't find out is if there's any disadvantage to a mini over a full-sized one. So far as I can see there are some advantages: tend to be cheaper fit in the CU effectively double pole don't need to be disconnected when running tests

What am I missing on this? What's the point of big RCBOs?

[Andy Burns]

I think the first generation RCBOs were taller, then they managed to shrink them to MCB size in second generation, so I don't think there's any 'point' as such to the tall ones.

[ARW]

The small ones often do not have the functional earth fly lead.

[PeterC]

Looking at the manufacturer's site for Wylex and Crabtree, neither do the big ones. The full-size ones mention disconnection before testing the circuit; the miniature ones don't.

[The Natural Philosopher]

No RCD/RCBO has a 'functional earth fly lead'.

[Andy Burns]

The white lead here ...

[The Natural Philosopher]

...is not part of the trip function. No idea what its for, perhaps to earth something in the switch...

[ARW]

It allows the RCBO to continue working and detect a LE fault in the unlikely event of a missing/lost neutral.

RCD's do not have these fly leads.

And wankers oversleeve the flyleads with green/yellow. But then some RCBO manufactures have a flylead with a green/yellow cable.

[Andy Burns]

Zoom in ... it's shown as part of the circuitry measuring current differences

[John Rumm]

It is - but it adds a capability that normal RCDs don't have.

It will allow it to trip in the unlikely event that you get a neutral disconnection. (which on a PME supply could leave all earthed metalwork in the house at mains voltage)

[PeterC]

Can that fly lead have a DIY equivalent wired into a mini-RCBO? I can't see from the image that Andy posted what it's connected to on the RCBO.

[Mathew Newton]

No. (It'd have to be connected internally to the imbalance detection circuit).

[williamwright]

Glad you came along Adam!

Bill

[The Natural Philosopher]

You haven't looked at the circuit diagram have you? Or thought about what you just said.

How is the current - the massive current - that should flow down the neutral going to get back to base if there is no neutral?

Down that poxy little wire to *earth*? What???? Earth is simply safety, it never passes significant current except under fault conditions. You cant operate *at all* without a neutral.

[The Natural Philosopher]

I did and its clearly there to earth a bit of metal in there.

THINK. How COULD it be part of it. If it passes any current at all it is CREATING an earth fault.

[PeterC]

I see - thanks.

[Roger Hayter]

All they have to do is draw the current for the electronics *before* the imbalance detector.

[The Natural Philosopher]

WHAT electronics?

[Andy Burns]

THESE electronics ... the white earth flylead seems to connect to a pin on the balance transformer (whatever the correct term might be)

[Andy Burns]

no, it might only go to the other circuitry on the PCB, not to the windings ...