safety cut off switch question please

Hi guys.
Had abit of a nasty today where l cut through the power cord on the circular saw - yep you heard right .
But - the power box safety switch didn't cut the power is there some reason why it wouldn't ?
lt's a brand new power box fitted with the latest but the house is 40 years old and the extension cord for the saw was running from the garage power point out the back . Maybe it doesn't work for the garage ?
The saw kicked back , l looked down and saw the cord cut clean in two , dropped the lot and ran , urr back to the garage to kill the switch .
On reflection now it really has me p'd off because l thought we were safe from an accident like that with cut off switches . Damn thing cuts off all the time around the house , even when a light bulb pops .

Cheers
MB

MB,
as I understand it safety switches measure any imbalance between the power in the active and neutral lines as this will be flowing to earth (possibly through your body:eek: ). There's a minute chance that you could cut through the cord and just have the active short streight to the neutral but that should trip the circuit breaker as you'd be overloading the circuit. Unlikely as it sounds you may have just cut clean through the cable without any shorting. I once cut through an underground cable that was feeding half a school. I was standing ankle deep in water using a large concrete cutter (wet steel blade). I cut clean through the cable, didn't trip out the power or even blow a hole in the blade - a bloody miracle. Maybe the same thing happened to you. If in doubt get a sparky in to test the circuit. You could try doing a test trip on the safety switch and see if your garage is covered by the safety switch first.

Mick

Quote:

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Originally Posted by masterblaster

the power box safety switch didn't cut the power is there some reason why it wouldn't ?
lt's a brand new power box fitted with the latest but the house is 40 years old and the extension cord for the saw was running from the garage power point out the back . Maybe it doesn't work for the garage ?

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By power box safety switch do you mean an earth leakage system.

Is the circuit to the garage on that circuit ?

If It is then you have hit the one in a million ( or more ) by cutting through the cord and not causing the safety circuit to lock out

There are ways to test but I would be hauled severely over the coals to advise them, so the advice is get a lecco in to test the circuit , electricity will zap you and can kill on the first encounter, not to be toyed with ever.
Rgds
Russell

As previous posters have advised you might(have) been very lucky.

Call an electrician to confirm that your garage circuit is actually on the RCD and that your RCD is functioning.

You might find something strange like:

1. Your garage is not on the RCD circuit.
2. Your garage is in an extension to a light circuit (not always on the RCD).
3. Your RCD isnt working.

The RCD is meant to measure minute differences in the returned amps about .02 milliamps and trip the circuit in about 0.2 of a second, i.e.before you get zapped - this is where you story should have been...the power went off and then I realised that I'd cut the cable.

A builder told me one of his aprentices cut through his power saw lead, the thing that had the builder amazed was his radio still stayed on lol.

I guess that kid was one in a million times two:eek: :D


HJ0 one day(now lol) i'll tell you about the forklift that dropped a full pallet, from max height to the ground. All the forklift experts said thats impossible even if hydralic line burst(back-ups etc) but we have heard of this happening once before lol.:rolleyes:


HJ0

There is a way for the un-qualified to test their safety switch circuit.

Available through Turks is a "power point tester" which also tests the residual current device - RCD (safety switch).

This little box first plugs into the power point and some little LEDs will glow and the pattern of the LEDs will tell you if the system if first wired correctly or incorrectly.

Once the point is found to be wired correctly from the LEDs you press the RCD switch and slide the trip load up until the RCD trips off.

If the point doesn't trip or if the pattern of the LEDs is incorrect then get an electrician to have it fixed. Do not attempt to make repairs - this device is only to tell you if you have a problem or not.

This tester is also good for testing all the points in your house to ensure that they are all wired correctly and and the earth is fitted and working.

Quote:

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Originally Posted by Markw

There is a way for the un-qualified to test their safety switch circuit.

Available through Turks is a "power point tester" which also tests the residual current device - RCD (safety switch).

This little box first plugs into the power point and some little LEDs will glow and the pattern of the LEDs will tell you if the system if first wired correctly or incorrectly.

Once the point is found to be wired correctly from the LEDs you press the RCD switch and slide the trip load up until the RCD trips off.

If the point doesn't trip or if the pattern of the LEDs is incorrect then get an electrician to have it fixed. Do not attempt to make repairs - this device is only to tell you if you have a problem or not.

This tester is also good for testing all the points in your house to ensure that they are all wired correctly and and the earth is fitted and working.

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This is a good explanation and a way to test, for the un qualified. If you do indeed have a problem, an electrician be called. He should have a calibrated test instrument for testing the circuits are within manufacturers specifications

It is important that you get your house tested to see if infact all outlets are correctly wired and attached to the RCD device. There are different types of "Safety switches" on the market.

I do know the cost of the "power point tester " available from Turks it may be cheaper and safer to call the local electrician.

How new is the safety switch, is it under warranty, maybe you should discuss it will the electrician who installed the device.

Can you name the safety switch and model number?

Do not use the garage outlts until the circuitry is checked out

Hey Mick someone was looking down on you that day. lucky you didn't fall down into the drink in fright when you realised and got fried that way hey .
Ashore as far as l can tell [ knowing sweat FA about electrics ] it has trip switches and reset buttons plus a little blue button . Everytime it does cut off l usually just press everything l can find or nothing happens and eventually the power comes back on.
Thanks allot for the help guys . l had one thought , maybe when the cut cord fell to the ground the wires just didn't touch onto anything would that stop it from tripping ?
As far as l can tell the garage wirings been updated to maybe at the time the power box was it all looks new , neat and covered with piping ,well finished.
lf l was cashed up l'd get it all tested right now but l do have to get an elec out in a few weeks for something else , l better find some extra cash and get him to look at all that as well .lt's not much use is it if it doesn't save the day at a time like that.
Cheers
MB

Quote:

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Originally Posted by Simomatra

This is a good explanation and a way to test, for the un qualified. If you do indeed have a problem, an electrician be called. He should have a calibrated test instrument for testing the circuits are within manufacturers specifications

It is important that you get your house tested to see if infact all outlets are correctly wired and attached to the RCD device. There are different types of "Safety switches" on the market.

I do know the cost of the "power point tester " available from Turks it may be cheaper and safer to call the local electrician.

How new is the safety switch, is it under warranty, maybe you should discuss it will the electrician who installed the device.

Can you name the safety switch and model number?

Do not use the garage outlts until the circuitry is checked out

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Hi Sam , dunno how old it all is , not very l don't think , looks brand new , l've only been here 9 mths but l'll see if l can find some ID numbers or something on it for you and post them .
You may have hit on something though . The elecs card is stuck to the box inside , if l ring him up complaining that it didn't trip he'll probly race out and check it all for free you think .
Cheers.
Tell ya what , abit jumpy today onec l've had time to think about it all , to top it of l have to use the saw . A new cheapy l had to rush out and grab to finish the work.
MB

Hey Sam.

l hope you check back l got those numbers for you .
lt has an HPM switch box with a blue saftey on of switch which is always off if it trips and also a small blue test button .
An EPM Nilsen 2600 meter section .
And power cor no' 7494994

Ring any bells ?

Cheers
MB

Your circular saw is more than likely double insulated.
If it has a symbol of one square inside another on the serial number plate then it is double insulated.
This is more than likely why it did not trip your safety switch.
Double insulated tools have no earth connected to them.
As Mick said before, safety switches trip when there is an imbalance between the active and neutral, ie the return path for the current is through earth rather than neutral.
As there is no earth cable on your saw it is likely that there was no return path for the current, therefore no current flow so nothing for the safety switch to detect.
If the live cable contacted the ground or you then there may have been leakage to earth which would have caused it to trip.

When the blade was cutting the cable it more than likely came in to contact with the active and neutral at the same time. This would have been a short circuit and there may have been enough current that it should have tripped your circuit breaker.
However, you may have cut through it quickly enough for the current to remain low, therefore no trip on the circuit breaker either.

Cheers, Jack

MB,
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.
If both conductors were severed at the same instant by a metal blade one would expect that the supply circuit breaker should have tripped for a short circuit created by current flow from active to neutral via the metal blade. This can take only milliseconds and the current flow can be quite high (because there is little resistance to current flow).
Now if your RCD did not trip and the supply circuit breaker did not trip my only conclusion is that the separation of conductors occured at the same instant and there was no contact with the blade, which just doesn't seem likely.
Clearly there needs to be some testing of that particular circuit to prove that it is protected by an RCD and has appropriate overload and short circuit protection.
If so then all I can say is that it's a bloody mystery.
If there is no RCD protecting the circuit then you are bloody lucky there was not a scary flash.

Regards from Adelaide
Trevor

Quote:

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Originally Posted by TrevorOwen

..................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...........

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Trevor,
I'm no electrician, but this doesn't sound right to me as it would "read" the same to the device as a switch that broke one side of the circuit only. (I can never remember if that's a single pole or single throw switch:confused: ) As I understand it the device senses any imbalalnce between the active and neutral. Ie: 5 amps being consumed by device = 5 amps in on active and same amount out on neutral. When something shorts out to earth, either through the earth lead or perhaps through a tinkering DIYer then perhaps it's 10amps in on the active side which then goes to earth so there's no corresponding flow out on the neutral side. I'm sure that JackE and Cliff and all the other electrically learned board members will be cringing at my oversimplistic explanation and no doubt incorrect terminology :rolleyes:

And now for a very simple electrical rule that I was taught many years ago: "Assume that it's live and you'll stay that way."

Mick

The switch, depending upon the type could be a single pole single throw or double pole single throw, some connect only the active, such as a light switch while the latter connects (and isolates) both the active and neutral, on circuits written up as a SPST or DPST, and it keeps going from there depending upon the application and functions required, but, in essence what you are saying was correct.

When we moved in to this place 10 months ago it didn't have an RCD so I had an electrician (my mate) install one. The circuitry and potential load meant that he had to install two safety switches, 1 for the house the other for the shed, so it's possible that yours may not be in circuit with the RCD.

When he installed the RCD on the house we hit a problem with an out side power point for the well pump. so he disassembled it and through lack of a new power point throughly cleaned out the ants nest that had taken up residence, he then rewired it. "Go and turn on the mains Dave" said he and as I was walking away he threw the pump switch. You should have seen the colour drain from his face as the pump started. Yes he had been working on a live cable.

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

Cheers

Dave

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.

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Originally Posted by davo453

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

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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

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

Quote:

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Originally Posted by TrevorOwen

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.

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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

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 !

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.

Quote:

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Originally Posted by titand

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.

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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.

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 :rolleyes:) tradespeople they have their place.

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.

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