Circuit Breakers and cut cables

[Biker1959]

After dropping the cable of my hedge trimmers onto the blades the 20amp circuit breaker in the house supplying the garage tripped, but not the 20 amp circuit breaker in the garage. Pics of house and garage attached. Should the garage have tripped first? View media item 67084 View media item 67085

 

[riveralt]

I would of hoped that your RCD would also have tripped
But in terms of your two MCB's its pot luck - one might be slightly more sensitive to the other.

 

[plugwash]

If you put two breakers of the same or similar rating one after the other than it's going to be pretty much random which one trips first. Even with breakers of very different sizes it's not unheard of for more than one breaker to trip in the event of a high current short circuit.

 

[JohnW2]

I would of hoped that your RCD would also have tripped

Well, you never know, the hedge trimmer may possibly have been plugged into an RCD socket or plug-in RCD

The OP obviousl ought to have RCD protection, particularly for outdoor tools. However, things like hedge trimmers are usually Class II, with 2-core flex, so unless the live cut end (or the blade which chops the cable) comes into contact with earth, one would not expect an RCD to operate when one chops through the cable, even if there were one!

Kind Regards, John

 

[ban-all-sheds]

Should the garage have tripped first?

As riveralt says, it's pot luck.

There's no point having that CU in the garage, BTW.

 

[ban-all-sheds]

 

[ban-all-sheds]

I would of hoped that your RCD would also have tripped

I think his RCD has gone on a trip, and not returned.

 

[Biker1959]

THANKS to all of you for your help. (The plug in RCD is now an ex RCD- it has ceased to be...)

 

[SaladFingersMKII]

Should the garage have tripped first?

As riveralt says, it's pot luck.

There's no point having that CU in the garage, BTW.

Why not? There's every point to having a CU to distribute to different circuits in a garage.

 

[ban-all-sheds]

Not when it's on a 20A supply from an MCB in another CU.

Sockets directly on the 20A radial, FCU for the lights.

 

[mfarrow]

At the house end, a 20A cartridge fuse would stand a better chance of discriminating with what's in the plug than a CB . This may stop the house side from tripping, and won't cost you much.

You also need less current to blow a 20A BS88 in 5s than to blow a B20 in 5s, so it shouldn't affect the circuit design detrimentally.