Lawn mower burst into flames. Residual protection?

[Nijinski001]

Okay so that was probably an exaggeration. I've told myself a millions times .... don't exaggerate.

On Sunday I was mowing the lawn when the cable entering the handle burst into flames. On checking it would appear that a cable snapped at a point where it was constantly being moved.
On checking the CU I noticed that the MCB had not tripped which to be honest I was surprised and concerned.

My question - I don't have RCDs at the moment and stupidly wasn't using an RCD plug in. As this plug is used for the lawnmower I would now like to replace the MCB for this circuit to a residual breaker. Which one would be more suitable for this purpose? An RCCB or an RCBO?

Ta

 

[ban-all-sheds]

RCBO.

But even if you'd had one, it wouldn't have done anything to prevent the fire - there was no leakage to earth.

 

[Nijinski001]

No but at least it would have tripped the RCD bit to ensure no electrocution. My concern was that had it happened to my wife she may have stuck her fingers near the damaged cable after the small flames had put themselves out.

 

[slippyr4]

No but at least it would have tripped the RCD bit to ensure no electrocution.

Not necessarily.

 

[Nijinski001]

Slippy - Can you explain please?
Surely the fire and break in the circuit would have caused an imbalance tripping the RCD?

 

[slippyr4]

Slippy - Can you explain please?
Surely the fire and break in the circuit would have caused an imbalance tripping the RCD?

RCD's detect an imbalance in current flow between live and neutral. In your (probably) Class 2 lawnmower, there (probably) won't be an earth conductor, and therefore what comes out the live (probably) is going back up the neutral, thus no imbalance.

If there's an earth core (even if its not connected at the lawnmower end) then you've got a much higher chance of the burning cord sending some of it's juice down the earth core, whereupon the RCD may trip.

Other scenarios where the RCD might do it's job are (possibly) if the burning cord is touching the ground (although this probably wouldn't do the trick) or if you were touching a live conductor yourself.

RCDs are important and useful safety devices, but they're not magic.

I'm only explaining in more detail what ban-all-sheds said above.

The key lesson to learn here is that you really ought to keep an eye on the condition of the cord to your appliances, and replace the cord if necessary. Better to prevent the situation happening in the first place rather than rely of safety devices. Lawnmowers, hedgetrimmers and irons are appliance where cord failure is relatively common.