safety cut off switch question please

[masterblaster]

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

[journeyman Mick]

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

[Ashore]

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 ?

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

[spartan]

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.

[HJ0]

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:


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.


HJ0

[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.

[Simomatra]

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.

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

[masterblaster]

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

[masterblaster]

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

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

[masterblaster]

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

[Jack E]

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

[TrevorOwen]

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

[journeyman Mick]

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

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

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

[Iain]

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.

[davo453]

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

Cheers

Dave