3A fuse blew before RCD trips

[Lobster]

OK this friend of mine was cutting his hedge last night, and managed to chop through the flex of his hedge trimmer. Tsk - what a plonker, eh? I'd never do anything as daft as that.

The bloke was using it on a RCD-protected cable reel, but was surprised to note that it was the 3A fuse in the trimmer's plug which blew, rather than the RCD in the cable reel's 'plug' which tripped. The RCD test/reset routine checks out fine.

Is this normal, or is the RCD faulty/not very good? Shouldn't it be more sensitive than a fuse?

David

[Dave]

I guess he managed to connect Line to Neutral before the Earth got involved ... or is it just a 2-wire lead?

Dave

[Andy Wade]

No, the fuse and RCD are there to do different jobs. I's guess that the trimmer was a Class 2 appliance (double-insulated) with a 2-core flex. Cutting through the cord - easily done BTW - would cause a L-N short circuit and blow the fuse. It wouldn't cause earth leakage to trip the RCD.

[David Hearn]

Sounds sensible - so, does chopping through a 2 core cable pose more of a risk to the user than a 3 core + RCD? Is there a realistic chance of electrocution with either? I suspect there is with the former if the live/neutral didn't touch but touched the operator, and not with the latter (or far reduced) - if this is the case, why are cutting devices provided with 2 core flex and RCD's are recommended for cabled roaming cutting devices?

Thanks

David

[tony sayer]

If its a two core and non RCD protected then there is a good risk of a belt. Depends which wire it hits and how, and of course how it unit is made if its fully insulted so what you're holding is insulated then very little risk unless its wet or damp.

As long as the trip goes off at its rated 30 ma or less, then in practice no..

anyone who uses any mains powered gear outside without RCD protection deserves what they get IMHO...

As above, depends on "how" you get connected to the live line when it cuts the cable!, and how damp it is and how you are connected to earth etc.

[bigcat]

With 3 core earthed appliance, if the earth wire is cut through, as is likely, the user can expect to get connected to live or neutral, risky. With a 2 core appliance the user is insulated from the mains, so it doesnt matter whats cut, the user dosent fry.

provided

safer and cheaper.

NT

[Andy Dingley]

Not really - hedge trimmers are dangerous anyway, so whether the blade is at 240V or not doesn't really make much difference to the hazard of something that''s already a powered finger chopper at the best of times.

I'd want one with insulated handles anyway. As the most likely accident is this sort of cable chopping, then what you really _must_ avoid is the body of the tool going live. As to whether the overcurrent or the earth leakage should trip first, then that's really only an issue if you manage to strim through a cable to expose the live conductor, but without shorting them both together.

[Dave Plowman (News)]

There's a good chance the trimmer is double insulated and therefore only has a two core lead. For the RCD to trip needs an imbalance of current flow in live and neutral - usually caused by one or other shorting or leaking to earth. If they simply short, through cutting them quickly, the fuse will go before the RCD trips.

[Andrew Gabriel]

I managed to cut the cable to my hedge trimmer into about 10 pieces, which I would have thought impossible until I actually did it. It was one of those telephone cord type coily cables, and it managed to get itself stretched just right and then flicked such that each turn of the coil went into the cutting teeth at the same instant. There were several more loops which were partially cut, as the cutting teeth are not synchronised to cut together.

This did trip the RCD, but I never felt anything, so I assume the current went through the tree.

When I replaced the flex, I used 3-core with the earth lead not connected to the double insulted hedge trimmer. When I thought about it, it seemed safer than 2-core in some failure modes.

[Doctor Evil]

I have a CU that was in a house for 3 months and taken out complete, about

15 years ago, being in my loft ever since. It has a ELCB, rather than an RCD. Are they outlawed now? I have never bothered to find out. There must be millions of ELCBs around.

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[Doctor Evil]

I have seen a number external sockets around. If I was wiring them I would have a switchless fused spur on the inside and a cable running to the external socket, which may only be just across the cavity in many cases. I assume the switchless spur is mandatory when wiring an external socket. This also would give another level of protection. RCD to the CU, switched spur at 13 amp and 3 amp fuse on the appliance.

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[Dave Plowman (News)]

You've not really grasped this electricity thingie, have you? Do you worry it leaks out if there's not a plug in place?

[Andrew Gabriel]

Voltage Operated ELCB's weren't part of a CU, so most likely you have something else, such as a standard RCD.

Voltage Operated ELCB's are not 'outlawed' as installations that conformed to the regs when they were installed are not outlawed. They can't be fitted now, but there's no requirement to remove ones which are still working. Finding a test transformer for one might be an interesting challenge, so some electricians might remove it on the basis that they can't test it. Also, bare in mind they are not for protection against electrocution, and they might get removed in the process of adding protection against electrocution.

[Doctor Evil]

...fresh from polish the cabers the brainwaves continue......he brainwaves full steam ahead.....

........yes he does say that.......yes he did....his electricity leaks all over his cabers....

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[Lobster]

Yes mine, erm, my friend's I mean, is double-insulated, and also has an interlocked switch on each handle, so actually is impossible to operate without a hand holding each handle all the time (which was I think the cause of the accident as on the old hedgetrimmer I^Hhe used to use one hand to keep the flex out of the way! It's quite neat, as in safe, but a bit of a PITA in practice as it reduced your effective reach considerably when up a ladder (which I suppose is also no bad thing from a safety perspective, even if it means now having to tackle the top of the acer from both sides rather than one!)

David

[mike ring]

Lobster wrote in news:2qq5e.1141$c4.35 @newsfe3-win.ntli.net:

What a good little chap your mate is!

I bet my mate, who might have a 13A or nail in the fuseholder, would find the RCD tripped first ;-)

That's why he always uses an RCD

mike

[James Salisbury]

My parents's help did one better than that, cut the clipper's flex. It was pluged in the garrage. It even triped the 32A breaker on the submain to the Garrage. Got a phonecall to say that the RCD in the garrage won't trip on the test button. He had also blown the 15Afuse in the Garrage feeding the sockets... The fuses must have been arcing when the CB triped. I wonder can the fault level be too high on a domestic instalation?

[bigcat]

There must

If you mean a v-elcb, yes there are lots of oldies still in use. If youve installed one, for reasons unknown, have you installed the 2nd earth that it needs in order to function? And why did you install it? (As if we dont know)

Probably more likely its an early current differential operated RCD, from when they were confusingly called elcbs.

NT

[Andy Luckman (AJL Electronics)]

Funny you should say that Dave, my partner kept complaining about me leaving the switches on with no plug in place. Finally she told me that she thought the electricity would fall out onto the floor. :-)

[Dave Plowman (News)]

Yes. It would need correct termination? ;-)