How Dangerous are Metal Bodied Powertools?

[Duncan A]

I can't say I ever heard of a horrendous reputation regarding metal-bodied power tools although there are obvious dangers. Double-insulated are no magic bullet though. If you bridge the insulation (i.e. air) between the casing and the commutator of a drill with iron filings entering through the ventilation slots, you may feel 50Hz biting you!
From personal observation I can categorically state that pretty well any air cooled electrical hand tool dunked in salt water whilst in use will fail very fast!
For the above reasons, hand power tools for use on-site in most countries are limited to 110V or thereabouts, giving a max 55V to earth.

 

[Eric The Viking]

Sam: I understand your point, but I also understand that the Wikipedia page is saying that 2x rated current is used to define the fuse's disconnecting "speed". In other words when a current of 2x flows through it (under fault conditions), it will disconnect according to the speed rating it has.

I didn't mean to imply it wouldn't blow at less than 2x, but that it is intended to blow at 2x in the stated time, and below 2x it will tolerate overcurrent before it blows.

The point being in practice, with power circuits, the fuse limits the fault to 2x the rating current (otherwise it would go higher). For fault conditions with less than 2x current it takes longer, which was rather my original point: earths need to be low resistance, so that the fuse blows (correctly) quickly.

 

[Sideways]

As an illustration of the point about fuses having characteristic curves (the length of time to blow vs the current).
There's a place I visit which has a 1.5kW table saw. This runs OK from a domestic 13A socket but the fuse blows regularly - about once every 6-8 weeks. It's a capacitor start induction motor with a centrifugal switch to disconnect the capacitor once it's started spinning (probably takes less than a second).
The reason for this behaviour will be the fuse "ageing" prematurely due to the current surge when the motor starts. This will be much higher than the nominal 9-10 Amps current on the motor name plate. Although large, the surges don't last long enough to blow the fuse but they will be damaging it so that after enough of them it finally disconnects.
The curves for the BS1362 (plug top) 13A fuse show that if it only lasts 1 second, the fuse should be designed to handle a surge current of at least 30A without blowing and tolerances allow that this could be more. The same curve tells us that a 13A fuse is expected to carry 21.6A continually without blowing.
In this case, there isn't a great danger as the machine is never used unattended and the plug gets checked for heat damage when the fuse is being replaced, but it is a nuisance.
As part of a planned rewire the saw will go onto a circuit with a blue CE socket and a properly sized circuit breaker to handle the surges.

 

[graduate_owner]

Regarding the defective sockets issue, you can buy socket testers quite cheaply - £ 12 ish upwards. You just plug them into a mains socket and switch on, and the arrangement of lights will identify a defective earth, detective neutral, or crossed L and N wires. Cheap, portable and easy to use.
K

 

[Rorschach]

The cheapest socket tester that do that are less than £2. I have one in my toolbag used almost exclusively to see if I have switched off the correct breaker but handy to check wiring too.

 

[Eric The Viking]

Regarding the defective sockets issue, you can buy socket testers quite cheaply - £ 12 ish upwards. You just plug them into a mains socket and switch on, and the arrangement of lights will identify a defective earth, detective neutral, or crossed L and N wires. Cheap, portable and easy to use.
K

Yup. They're brilliant.

The triple-neon ones were part of our location toolkit and using them was part of our site survey routine. They found the live-neutral reversals I mentioned earlier.

The sockets I am really wary of are any rarely-used ones in older public buildings, from experience they can be quite nasty. That's not to say others aren't dangerous, obviously - with those testers there's really no excuse for not checking anything and everything you connect to. It literally takes about 10 secs, of which 6-7secs is crouching down so you can see the neons properly (old age!).

 

[Phil Pascoe]

That would be as selling point actually, a front panel angled at 45 degrees - how often is socket at eye level?

 

[Rhyolith]

This is all very interesting, thanks for all the responses!

Now, whats an “RCD”?

These tester sockets sound like a no brainer purchase, though not sure I’d trust one that costs £2! Can someone recommend one I can rely on? And do they do the same thing as the multitester, or is no necessary to have both?

 

[Rorschach]

It's not some piece of ebay rubbish from China. (prices gone up a bit since I got mine)

http://uk.farnell.com/pro-elec/t203/plu ... 0000055033

 

[AES]

I have to be a bit careful answering electrical Q's here, especially after making a right silly person of myself a few days ago! But AFAIK,RCD stands for Residual Current Detector - in other words if an internal fault occurs, some current may "leak" away - normally to earth rather than through you to earth. Also I THINK the same things used to be called ELCB (Earth Leakage Circuit Breaker) though I stand to be corrected on that one.

Either way, my understanding is that these devices are latched ON (i.e. power flows to the tool or whatever when there's no fault, and there's no "leakage" to earth) but if a fault does occur and a "leak" current of more than 30 milli amps is detected, then the latch is released and the whole thing (tool, cable, and the plug you've pushed into the wall) becomes dead. A VERY good idea IMO.

BTW, since the subject of those UK plug in fault detectors was raised here a few days ago I've looked around but not found any here so far. I'll keep looking but if I still can't find one I'll buy a UK version through amazon or someone and use it here with a normal Swiss plug/UK socket adaptor.

They too seem like a very good idea as even though I don't do any site work, it's by no means uncommon to find the "live" and "neutral" wires reversed - unsurprising really, as several different makes of plugs here do NOT have the terminals marked at all, just the earth. Dunno why.

 

[Sideways]

To answer the plug in socket tester question, they are a useful tool because of their ease of use. They perform a quick and useful check of the wiring to a single phase 230v mains socket and using simple adapters, they can be used to check blue 16 and 32 amp industrial sockets too.
If I was recommending brands, Martindale, Fluke and Kewtech (also branded as Socket & See) are all good brands. I own a Martindale and can vouch for their customer service as some of their models had a defect and Martindale carried out a massive recall a couple of years ago, replacing all of them with new ones, no question.
Price wise, they go up to about £50. The more expensive ones perform a measurement of the earth loop resistance that cheap models don't. A traffic light display gives you some confidence that the earth is a low enough resistance to quickly blow the fuse in case of fault.
These are testers. They can't make the measurements that a multimeter can and they are no substitute for the £350 - £1000 test meter that an electrician will use to check your house wiring but they are safe, easy to use and to understand. A cheap multimeter may be none of these things.

 

[Eric The Viking]

@AES: I know Swiss plugs aren't reversible, but Schuko ones are (usually - it does depend on the variant*). I'm trying to remember, but I believe the German wiring standard that allows reversible plugs mandates RCDs on each circuit, so theoretically it doesn't matter much if you swap live and neutral. It's a handy feature with the right-angle plugs as you can run the cable out upwards or downwards.

You could use a UK tester simply with a UK socket to Swiss plug adapter, but if you have Schuko sockets in the house (I've seen them in Switzerland, but I also know they're deprecated there), the live-neutral reversal won't be meaningful!

E.

*IIRC the French version has an earth pin as well as the earth connectors down the sides, making an Isosceles triangle between the pin positions, like the Swiss ones have.

 

[Rhyolith]

Am I right in saying there are two types of these socket testers? One that tests the earth in the actaul socket and another that plugs in with the tool and turns off the power if theres a fault (leakage)? Think I have seen the latter in DIY stores.

I assume you need to be sure the earth in working for the shut off version to work?

 

[Rhyolith]

Another thing, how do you test the earth wire on the tool itself is definatly working?

 

[Rorschach]

Another thing, how do you test the earth wire on the tool itself is definatly working?

Continuity test between the earth pin and the body of the tool.

 

[Duncan A]

Since we're talking about safety, the conductors are not "live" and "neutral"; they are "line" and "neutral". Both conductors are live in the sense that they conduct current. The terms "line" and "neutral" derive from the 3-phase supply which is used to provide a 1-phase supply to most domestic properties. Unless, of course, you're lucky enough to have a 3-phase supply!
Duncan

 

[Rhyolith]

Another thing, how do you test the earth wire on the tool itself is definatly working?

Continuity test between the earth pin and the body of the tool.

Is that what you need a multimetre for?

 

[Phil Pascoe]

If you haven't a multi meter, make sure you buy one with an audible circuit tester - they're much easier to use. You can hear when a circuit is made when you can't necessarily see the display.

 

[Sideways]

Another thing, how do you test the earth wire on the tool itself is definatly working?

Continuity test between the earth pin and the body of the tool.

Is that what you need a multimetre for?

Yes.
You use the "resistance" sometimes called "ohms" setting.
With the tool unplugged, touch the tips of the multimeter leads together to make sure that it reads zero properly before you start.
Then use it to measure between any point on the metal body of the tool and the metal of the earth pin on the plug - you may need to push the points of the leads into the metal to get a good contact.
Regardless of whether it goes beep or not you're looking for a very low reading - if the earth connection is in good condition it should be less than one ohm. If it's more than two then the connections are probably dirty, loose, oxidised or otherwise sub standard and should be renewed. Start by rewiring the plug and test again.
Some testers will beep to indicate continuity when the resistance is too high for an acceptable earth connection. Learn how your meter works before you assume that "beep" is OK for whatever purpose you're measuring.
Multimeters are a useful tool but they are exactly that - a multi function device - you have to know what to measure, how accurate they are and what is a "good" reading. The meter won't tell you and a lot of them won't protect you from user error - like forgetting you have it set to measure amps and then putting the probes in a live socket to check the mains voltage

 

[DTR]

- like forgetting you have it set to measure amps and then putting the probes in a live socket to check the mains voltage

I've never done something as silly as that before :-"

..... and I've certainly never had to explain to my boss afterwards why the fuse mysteriously blew without warning........ :---)