Once again I've managed to chop through the flex on my hedge cutters. They're rated at 400 watts, so I guess a 5amp fuse should have been used in the plug instead of 13 amp. Even so, I don't understand how the fuse survived what was a clean cut. Yes, I did unplug it instinctively, but I didn't see any urgency to do so because I assumed the fuse had blown.
Was it on an RCD? Did that trip?
I suppose its possible (but unlikely) the individual conductors are cut one by one without shorting them together, more likely the cut was done quickly enough that a short-duration short-circuit didn't have chance to blow the fuse, and the cut was clean so didn't leave a short after the cut was done.
At the moment of the cut the voltage must have been close to zero and/or the cable did not make contact for long enough to zap a fuse. Or did an ELCB or RCD somewhere else trip first?
It is unwise to ever assume that a cut power cable is not still live.
Both the above. Also 400 watts may be the usual running power but start up might use more. Suggest you retain the 13a fuse or use a 10a if you have one. A RCD or ELCB should be used if the circuit is not protected by one.
Yep - done the same with the lawn mower a few weeks back. It was on a 10mA RCD, and that didn't have time to trip either.
more likely to trip an RCB these days.
not enough i squared t to blow it.
Much will depend on the prospective fault current... this will be dictated by the supply impedance to your house, the circuit impedance from your CU to the socket in use, and then more significantly by how many metres of relatively skimpy flex were on the trimmer (and any extension lead). You would probably find if you did the sums, it all adds up to something north of a couple of ohms. That means that even under short circuit conditions you are limited to around 100A of prospective short circuit current.
Have a look at:
- and that assumes the short was there for a full tenth of a second.
Never a safe assumption (and another reason why using a RCD protected circuit is a good idea)
Erme - wouldn't it also be a good idea to never assume that an RCD has tripped?
Nick
Because it was exectly that - a clean cut.
Foolish asumption.
MBQ
Indeed it would.
I was not intending to give the impression that the RCD would cut off the power as a result of the cut - in fact its quite likely it would not since the tool is almost certainly of a double insulated design with no earth connection. So unless the blades also had a decent connection to earth at the time of the cut, the RCD would be unlikely to see enough imbalance to trip.
However the key point is that if as a result of assuming the cable is dead when its not, you then handled the live end, the RCD would keep you alive.
Thanks everyone. We live and learn
No, it wasn't. Just a normal extension lead straight into a socket. As it happens the fuse box in this property is an old fashioned type with no circuit breakers, so I will continue nagging them to get that upgraded.
If you're prone to pruning cables you could get a plug-in RCD, or extension reel with one built-in for outdoor use ...
Don't know whether there is a better way but I always run the cable over my shoulder to my hedgecutter. So far I have not hit my cable. It makes it easy for me to see and the cable does not hit the ground and cause the slack to go all over the place.
Twice in 40 years, and both in the last month. Oh dear
Let me rephrase that then, if you're /becoming/ prone :-P
However, had the OP then touched the still live cable end, the RCD would have served its purpose.
Did you really need to snip the bit where I said:
"However the key point is that if as a result of assuming the cable is dead when its not, you then handled the live end, the RCD would keep you alive. "
just to make that comment? ;-)
In which case change the plug on the end of your extension lead to a RCD type, then use that for anything that needs power outside.
Don't wait for someone else!
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