safety cut off switch question please

[Iain]

In the first year of my apprenticeship I went with my boss to a hospital in Perth to pick up a faulty defibrilator, it was discharging by itself but no one told me.
I picked the unit up and grabbed the pads as it discharged, we were in a cardiac arrest area at the time and I recall very little of it, the boss left with the faulty unit about ten minutes later, I left the following day after having another shot of defibrilator that was not faulty.

[Markw]

This guy is a very experienced consultant electrician, he swore me to secrecy but I know it will go no further than us few

Consultant he may be, sufficiently experience I don't think so.

It really surprises me the number of electrician who fry themselves or have near misses that would have resulted in the same.

An old Aussie movie called "Smiley gets a Gun" quotes "The guns always loaded and the mule always kicks"; well "the wire's alway live" too. Where I work our safe work methods have to prove that this isn't so and we go to extreme and expensive lengths to do this cause the wire next to your head may have up to 132,000 volts. That is not survivable. The HV mains are continually earthed for the work duration to ensure that nothing can happen.

For ten bucks your electrician purchases a cheap neon circuit tester. Plugs into the next circuit to prove the unit works, tests the "dead" circuit, retests a live circuit to again prove working condition. All over in less than a minute.

There are no excuses. This is why you were sworn to silence. Any electrician who doesn't prove their circuits before commencing is lazy or stupid or both and none of them should be re-invited to work on your property. Calling for the ambulance for a DB is not fun.

Sorry for the rant

[davo453]

absolutely agree and he said as much as he lamented his stupidity. I suppose the only excuse (such as it is) is that it was at the end of a very long day, but thats my supposition not his.

This guy is seriously Mr Safety and his job is just that to advise on safety. I guess even the best of us are (sadly) human and inherently make mistakes, if anyone makes out that they don't, they are kidding themselves or have short memories.

This guy was trained (30+ years ago) to treat all wires as live and this was why he didn't get zapped, the RCD had already been installed so that would have at least saved him to an extent, if it had needed to operate.

Cheers


Dave

[Jack E]

Let's assume your circular saw is double insulated and therefore the connecting lead has an active and a neutral conductor.
Let's also assume that your supply that you were using was protected by a residual current device.
If the lead was cut by the saw and firstly only one conductor was severed, the RCD WOULD or SHOULD have tripped. This is because the device operates in milliseconds for only milliamperes imbalance between active and neutral.

The RCD will only trip when there is an imbalance between active and neutral.
Just because one conductor was cut doesn't mean there is an imbalance.
If you cut the neutral and there is no return path for the current flow, then there is no current draw.
If there is no current draw then there can be no imbalance.

Cheers, Jack

[masterblaster]

Thanks again for the replies guys l'll explain exactly what went on it might help.
lt was an old Hitachi saw , bought at cash converters yrs ago . Rough but jesus you couldn't stop this thing - unless you cut the cord that is .
lt did only have 2 wires in the cord , it's down the tip now but this l clearly [very clearly] remember as l lifted the saw up gawking at the chop chop. This was a split second before l leaped 2 mtrs straight up .
l was taking an edge of a door layed flat across two stools , l always take allot of care with saws because l don't like them . But just this once l sort of went to restsrt the cut without taking that much notice and it kicked back and bounced , when l lifted it up here's the cut cord staring at me . The other end fell onto the ground . But youknow how quickly they can kick back so the whole thing was about a milli second and l was outa there.
But - just saying my trip switch does work on the shed , if those wires touch me would it have tripped then if it was all working properly ?
Or - maybe it's like Daves and the shed isn't even on that circut but if it was , bloody handy seens as that's the power l use for all the tools .
Cheers
MB.
PS - how do you guys use saws and things safely all the time is there some trick or proceedure to working safely even if your guards down for a second like this time .
l even wrote 'safety first' in red on the saw guard to remind myself but there ya go , just this once l get abit sloppy !
You see tadesman using saws one handed and stuff all the time but then l spose that is their job with years of practice so , me it's both hands on everything , but again there ya go this was a one of one hand time !

[titand]

MB,
If there were only two wires then Jack E is right, the saw was probably double insulated. The simplest way to tell if the garage is on the RCD (safety) circuit is to press the test button on the safety device, if you lose power in the garage then it is.
The test devices MarkW mentioned are very good, in fact alot of sparkies use them because it can save alot of time, but you should understand what its testing for.
If the wire ends did touch you the RCD should trip if there was a dangerous current flow though you body to earth, however you could still have received a zap and burn if current flowed from one wire through your finger to other.

You probably cut the cable so fast that there wasn't enough excess current for long enough to blow the fuse.... however recently I was removing a broken power point and tried tripping every circuit on the fuse board to isolate it but none did. The only way I could isolate it was to open the main switch (or RCD). When I looked further I found the circuit came directly off the mains side of the fuse board ie. No fuse! The fuse board, wiring and RCD were relatively new but somehow, someone made a mistake either cos they weren't concentrating or didn't understand what they were doing. One faulty appliance could have burnt the place down. However the safety switch would still have protected everyone from a shock!

MB, I'd suggest for your own peace of mind you have an electrician check it all out. I'm fine with electricity, but when it comes to gas I'll let someone else do the worrying.

[Markw]

The test devices MarkW mentioned are very good, in fact alot of sparkies use them because it can save alot of time, but you should understand what its testing for.

Titand, the reason I use this device, BTW it's a Digitech QP2000, is that it quite quickly tests for a mirad of problems including incorrect polarity or connections at the point or elsewhere in the circuit, unconnected earth, unconnected neutral using earth as neutral etc. All in all 6 different configurations and only one of them is correct and safe.

Then the last function is to test the safety switch. The internal RCD test point won't provide you with a sliding scale of milliamps to trip the RCD which is critical to whether you survive a contact situation or not. The test unit has a slide switch starting at 10mA, then 15mA, 30mA and 100mA. If it trips at 10mA then its probably too sensitive. If it trips at 100mA then its too insensitive and you might not be survive (elderly, pacemakers, children, etc). If it doesn't trip its broken. I have ensured that all mine trip under the 30mA mark.

This is a very good device and gives me reasonable peace of mind that it all works as it should.

[titand]

Hi Mark w,

I absolutely agree, the tester methodically tests for a wide range of problems quickly and reliably, and you obviously understand what problems its testing for and what actions to take to fix the problem. I think its invaluable and should be a standard tool, just like a multimeter.

What I was suggesting was that MB may gain more peace of mind by having an electrician check out his home because with all due respect he may not understand what the tester is telling him. While there the electrician can also check for other potential problems that most testers can't detect, like unfused circuits and wiring or insulation in poor condition or incorrectly sized. If MB is confortable with the results from a tester then that's fine, I don't mean to suggest we should call an electrician to change a light bulb, but like all (or most ) tradespeople they have their place.

[Markw]

Titand
I couldn't agree more. If in doubt get a professional or have a professional formally teach you what to do.

Electricity doesn't take prisoners and you won't hear the shot that kills you.

[masterblaster]

Thanks again guys .
l don't think l mentioned that the other side of the cut cord must have just fallen straight to the ground . How it didn't brush my leg on the way down l'll never know . l think it might have but side on , with the clean cut the wires would have been facing ahead and so not contacting my leg .
Think l will have to lash out on a pro though because l wouldn't have the knowledge to test it all myself or feel confident about it.
Something like this really wakes you up at how important thing's you might not be bothered with normally , can be .
All the best and thanks again.

MB