Testing for dead - Page 1
Forum section: electronics / projects

author: blackman
author_rank: Contributor
timestamp: April 18, 2012, 10:41:43 am
content: Hello fellasI work in the Coal mines in Central Queensland and being quite a big industry you can imagine the safety involved in day to day tasks.One thing we have come across is the good old "Test for dead" and how we can make it as safe as possible.There have been a few 'discussions' about this over the past weeks and I thought I would get your input.So at the moment our Test for dead goes something like this.Firstly sparkie to wear full PPE and a 40cal bomb suit. Safety observer with LV Rescue kit gloved up and with crook in hand, second safety observer standing at isolation point upstream if too far away.1. Isolate circuit.2. Open cubicle.3. Test multimeter.4. Test circuit, Phase to Earth then Phase to Phase.5. Test multimeter.The main point of discussion we have had was about testing the tester. How do we ensure with out a doubt that what you used to test the tester has worked. Especially when sometimes you are testing something that has been isolated and there is no power out there to test your tester. And also the safety involved in opening a live board to access live conductors for the test.We have found a device which generates a 240V AC signal from some batteries, its current limited and obviously only good for high input impedance meters, which is most meters that you would use for a test for dead.Some people have said, why do you just use a battery for a voltage, or short the leads and see if they have continuity, both of which I don't believe are good enough to prove your VAC range is working correctly on your meter.The other thing which was raised was the frequency at which you are testing the meter, Im not sure at which frequency this device is producing the 240vac, and how much does that matter to the test?Personally I think its a great idea and I think its going to become mandatory that everyone has one and everyone uses it for testing instead of using anything else as the fault current of this device is <10mA where in a board you can have anything from ~1000A to 50kA

author: Rerouter
author_rank: Super Contributor
timestamp: April 18, 2012, 11:19:53 am
content: they use a tester because a strong AC signal cannot be misread easily, and an ohms check is a seperate circuit to the VAC measuring, still, you would want the device itself to clearly show when its live, but why only 240, doesnt it also need 415, 1KV, 3KV and 11KV for mining,and yes frequency matters,(i have edited this quite alot i am sorry if I trick any future posters)

author: Unknown
author_rank: Unknown
timestamp: April 18, 2012, 11:29:32 am
content: Quote from: blackman on April 18, 2012, 10:41:43 amVoltage at any frequency including DC violates the "dead" condition. Therefore a meter must respond at any frequency including DC.QuoteI think (personal opinion) that even a minuscule amount of charge is enough to test the functionality of a meter. If such a device is built inside a meter it can be used for self-test and also to test another backup device.Those hand lights with a coil and magnet inside that you need to shake in order to charge come to mind.Hold down a test button, shake a few times, if a light comes on and the meter shows a reading it is good. No light - no good.It would be easy to operate, does not rely on batteries and can be completely sealed and ex-proofed.I believe the entire thing can be made as small as AA battery.

author: Rerouter
author_rank: Super Contributor
timestamp: April 18, 2012, 11:33:46 am
content: problem with the hand lights is it would need to maintain a constant ac or dc voltage for upto a minute (some mines really go nuts over safety)

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 18, 2012, 11:44:49 am
content: I've seen standalone 'proving' units to test meters like this, but it would seem a good idea to acually build these into the meter - couple of holes into which you poke the probes.Anyone seen this on a meter?

author: Unknown
author_rank: Unknown
timestamp: April 18, 2012, 11:53:45 am
content: Quote from: mikeselectricstuff on April 18, 2012, 11:44:49 amNo idea how to reliably ex-proof contact points. I was thinking about completely potting the thing in epoxy and having the test button operate a reed switch with a magnet from the outside. As long as the test button is pressed the charge is stored in a cap and powers the test circuit. Releasing the button discharges the cap through a 1Meg resistor.

author: Bored@Work
author_rank: Super Contributor
timestamp: April 18, 2012, 11:58:15 am
content: The first thing I would suggest is to dig through all the rules and regulations you surely have and to check if there are mandatory checks for the tester or if there are some best practice suggestions. This would be the minimum you should follow. On top of that you can establish your own procedures. Nothing worse than being caught with having substituted an official procedure with your own.Second thing I would suggest if you don't find something in your regulations is to look how it is done elsewhere. Not necessarily how it is done in China, but somewhere where they do care about safety in mining.What worries me is the possibility of sparks. If you bring an additional AC source into the mine, and someone happens to short out the outputs, creating some spark, and being in the wrong place ----> kabooom.Now, as a layman in mining, and rejecting all responsibility for applying the following idea, If you can control this issue, my idea for your would go in the direction of a small handheld, hand cranked generator with a gearbox and mechanical break to make sure you roughly generate the required AC at the required voltage. Connect the tester, crank the handle a few times while watching the tester.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 18, 2012, 12:11:25 pm
content: Dont really want to go back to the dark ages, but if you are only testing for dead, the old electricians globe was all right,1. Isolate circuit.2. Open cubicle.3. Test globe.4. Test circuit, Phase to Earth then Phase to Phase.5. Test globe.But you can actually validly test the AC response with a fairly low voltage dc just to see if it glows a bit.No chance of incorrectly switching/connecting the multimeter either.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 18, 2012, 12:26:32 pm
content: Quote from: HAL-42b on April 18, 2012, 11:53:45 amDon't think that would be a problem as you could have contacts deeply recessed down holes just big enough for the probes, current-limited, and maybe also switched so it only turns on when both probes are pressed into the holes.A more difficult practical issue is that anything would probably need to be certified, which an expensive process.Of course another approach may be to simply use two meters for redundancy.

author: IanB
author_rank: Super Contributor
timestamp: April 18, 2012, 02:04:56 pm
content: Quote from: blackman on April 18, 2012, 10:41:43 amTest the tester on a known live circuit in a safe place outside the mine before entering?As Mike said, why not use two testers? Both testers have to agree or the result is not trusted.Lastly how do you make sure the circuit remains de-energized after the test? I think in other safety critical situations they may apply a solid short circuit between all phases and earth. This ensures the potential of the conductors cannot rise above earth potential even if something goes wrong with the lock-out at source.

author: Unknown
author_rank: Unknown
timestamp: April 18, 2012, 03:12:30 pm
content: Having two devices may work but how many hands do you need to cross check two devices with 4 leads while wearingfull PPE and a 40cal bomb suit(whatever that might be)? All that in a poorly lit damp and dirty potentially explosive environment? It might work but don't expect sparky to buy you a beer for making his life easier.I think in those conditions every single device should be capable of absolutely positively proving itself safe, on the spot, any time.Safety check should not rely on external reference or consumables like batteries.Should be safely operable with one hand leaving the other hand free.Totally intrinsically safe rugged IP67 etc etc..Here is what I was trying to describe previouslyNormal two contact tester capable of testing DC as well as AC.1- To self test the instrument the TEST button is pressed and held down.2- The instrument is shaken in the hand 3 or 4 times while holding down the test button. A green TEST OK led will light up and the meter will read precisely 1V.3- To test another instrument touch the leads to the leads of the other instrument.3- Releasing the TEST button will cause the led to go off and the voltage reading to return to zero. Meter is ready for use.Pressing the TEST button causes the reed switches to change state. S1 and S2 N.O. switches close and connect the test circuit to the meter probes, S3 N.C switch opens and thus disconnects R3 shorting resistor from the circuit.Shaking causes the magnet inside the empty tube to move up and down. This induces EMF on the coil wound around the outside of the tube. The coil produces electricity which is rectified by the bridge and stored in a cap. When the stored voltage is high enough the led will light up. The zener will clamp the voltage to 1V (another diode will be needed in order to prevent the zener from discharging the cap down to 1V).Releasing the TEST button causes S3 to short the test circuit across R3, S1 and S2 disconnect the circuit from the probes.If any of the reed switches does not work either the meter wont read 1V or the green led wont light up.

author: G7PSK
author_rank: Super Contributor
timestamp: April 18, 2012, 03:31:36 pm
content: Why not use a moving coil meter like the old AVO panclimatics gas, tight robust and having no electronics less likely to fail. Carry two or three use each in turn. I assume that all your switch gear has lock outs with multiple locks so that you can be fairly sure that the circuit is dead as long as you have turned the correct switch. Under these conditions you are more likely to get hit by an asteroid than find a dead circuit live. So several meters of whatever type should be enough redundancy for all safety purposes.

author: Rufus
author_rank: Super Contributor
timestamp: April 18, 2012, 04:15:54 pm
content: So why don't the 'panels' have hard wired indicator lamps which you verify are on before and off after isolation?

author: Unknown
author_rank: Unknown
timestamp: April 18, 2012, 04:32:19 pm
content: Quote from: Rufus on April 18, 2012, 04:15:54 pmSuppose a piece of heavy equipment crashed into the panel and broke all lights panel meters and switch handles. You still need to check for dead before attempting repair.

author: Rerouter
author_rank: Super Contributor
timestamp: April 18, 2012, 08:24:58 pm
content: not to mention lamps are a potential ignition source, and even wit leds you then need a way to confirm the led is working and then that the driver for the test circuit is working,

author: Rufus
author_rank: Super Contributor
timestamp: April 18, 2012, 11:08:55 pm
content: Quote from: Rerouter on April 18, 2012, 08:24:58 pmWhat part of "verify are on before and off after isolation" do you have trouble understanding?

author: Unknown
author_rank: Unknown
timestamp: April 18, 2012, 11:34:38 pm
content: Quote from: Rufus on April 18, 2012, 11:08:55 pmWhat if the lights do not turn on at all no matter what you do?What do you do then, assume safe?

author: Rufus
author_rank: Super Contributor
timestamp: April 19, 2012, 03:00:27 am
content: Quote from: HAL-42b on April 18, 2012, 11:34:38 pmOnly if you are incredibly stupid - why do you need to ask?

author: quantumfall
author_rank: Regular Contributor
timestamp: April 19, 2012, 07:32:28 am
content: Are we talking about installations that cannot be isolated because their feed is unknown or unsure ? If so thats the real root of the problem.I'm sure you must have a formal procedure thats agreed to be legal by now ?At a guess two or more high quality meters different designs to avert common mode failure , designed for the worst case environments you will use them, tested immediately before use or as soon as possible, in a safe place, with accelerometers built in to warn of any adverse shocks or handling before the test, in my opinion thats what will cause failure in a short time is rough handling. Have a self test built into the meters as well.If it was me I would only feel confident knowing the system you are working on and how its isolated physically.You can only go so far, the system of isolation is your best defense, if thats not optimal, thats your problem IMHO.