RCD connected mower, cut through live cable, should this happen?

[choppachimp]

13A 2-Gang RCD Switched Plug Socket Metal-Clad, connected in the garage as part of the MCB managed plugs. Think they are a spur rather than a ring.

this morning, I finally did the very thing I had had the sockets installed to protect against

Should the 13A fuse in the mower plug have blown? (guess it's the first point of call)
The RCD tripped? (that much I was expecting)
AND the 10A MCB tripped? (that surprised me as it was before the RCD in the circuit)

am I wrong in assuming the the RCD would be the only thing to trip when you cut a live cable?
(its not something I've tried to do before) or is it the way the circuit fails that causes the various protections to ALL do their thing?

end of the day it saved me from death-by-worlds-smallest-mower

 

[AndyPRK]

If the brown wire and blue wire short together I would expect the MCB to trip. (and fuse blow if MCB didn't)

If the brown or blue come into contact with earth or human, hopefully the RCD will trip

 

[choppachimp]

ahhhhh THANKS for that Andy!

you know what; thinking about it I can't be 100% sure if the RCD did trip,
but the fuse and MCB certainly did.

the mower made a meal of the live cable and after a brief spark it all stopped.

Was just surprised the MCB tripped, which is beyond the RCD in my eyes....

but from what you say, perhaps no earth or human was the reason (think the mower is only 2 core)

 

[AndyPRK]

Oh yeah, they tend to be 2 core.

So if they shorted together the MCB would have gone before any earth leakage

 

[winston1]

RCDs do not trip on overload, that's what fuses/MCBs are for. RCDs trip on unbalanced loads usually due to earth leakage.

With fuses/MCBs in series the lowest value or quickest one will fail first MCBs are quicker than fuses.

 

[Taylortwocities]

MCBs are quicker than fuses.

Especially as (if I understand the OP properly for once) this has a 13A fuse in the mower's plug and a 10A MCB protecting the circuit.

 

[Robin0577]

RCD sockets sometimes have non latching RCDs, so the RCD may simply have dropped out to to the lack of power rather than having been tripped.

 

[JohnW2]

RCD sockets sometimes have non latching RCDs, so the RCD may simply have dropped out to to the lack of power rather than having been tripped.

Indeed., In fact, as far as I am aware, all RCD sockets (and 'RCD adapters') have 'active' RCDs (i.e. will 'trip', and stay tripped until manually reset, if power is lost). ... at least, I've personally never seen an RCD socket (or RCD 'adapter') with a 'passive' one (like we have in CUs).

Kind Regards, John

 

[EFLImpudence]

https://www.screwfix.com/p/mk-masterseal-plus-13a-2-gang-switched-rcd-socket-passive/76855

 

[JohnW2]

https://www.screwfix.com/p/mk-masterseal-plus-13a-2-gang-switched-rcd-socket-passive/76855

Rules wouldn't be rules without exceptions 'to prove them'

Kind Regards, John

 

[SUNRAY]

I have often found neither trip as the cable gets cut so quickly it's below the tripping time and no earth leakage occurs.

 

[JohnW2]

I have often found neither trip as the cable gets cut so quickly it's below the tripping time and no earth leakage occurs.

I gave up any 'betting' on this sort of thing years ago ...

... I'm sure that I must have reported here before the time when I chopped the (2-core) cable of a hedge trimmer - the 13A fuse in the plug, an B32 MCB and an RCD didn't blink an eyelid, but the 60A or 80A fuse of the submain supplying it blew!

Kind Regards, John

 

[SUNRAY]

I gave up any 'betting' on this sort of thing years ago ...

... I'm sure that I must have reported here before the time when I chopped the (2-core) cable of a hedge trimmer - the 13A fuse in the plug, an B32 MCB and an RCD didn't blink an eyelid, but the 60A or 80A fuse of the submain supplying it blew!

Kind Regards, John

Been there too, rack lighting in telephone exchange, 12x5A fuses in board, with somewhere around 10or12x40W florry's per fuse. A choke in a fitting cooked and took out the 60A sub.

a single 500W stage lamp blows when powered from darkness.
4A fuse in dimmer, 13A fuse supplying dimmer, 16A MCB radial, all stay intact but the 63A sub trips.

 

[JohnD]

'to prove them'

"Prove" means "test"

Hence "the exception proves the rule" (and shows it to be wrong)

The proof of the pudding...

Proof of a gun...

Proof of whiskey....

 

[JohnW2]

"Prove" means "test" ... Hence "the exception proves the rule" (and shows it to be wrong)

You're 'spoiling' it for 99%+ of the population, who believe that "the exception which proves the rule" means something different from that (which, in terms of use of language by that 99%+, it does!)

Kind Regards, John