Anyone ever had a false positive testing for dead with...

[JohnW2]

Edit: My eX rated meter does not respond to AC on a DC range nor vice versa and it was not cheap!

Maybe I didn't pay enough, then (many years ago!) ...

Untitled

• JohnW2
• 18 Nov 2011
• 1

The reading is a bit low, but that's to be expected because the meter adjusts so as to read RMS (not peak) values on an AC range.

Kind Regards, John.

 

[efiste2]

At Scot Power we have had a batch of Schneider 11kv Ring Main Units that could possibly still be switched ON when displaying off, The problem came to light when an Engineer switched one end of a cable to OFF, or so he thought, and then proceeded to switch the other end into EARTH..... SO for know if making an HV cable (fed via one of the above switches) dead to work on before any EARTH is applied it has to be proven dead with a SPIKING GUN...

 

[RF Lighting]

Mine doesn't...

 

[ericmark]

I have had a cable proved as dead become live during the work of removing it. A junction box from a removed batching plant still had one cable which feed an augur going through it. During the test the augur was not running so proved as dead. During removal the augur was activated so cable became live. After that it was decided cable would only be removed when plant was not in use.

As to proving units I found many only give 500v which of course lights up all warning lights but does not prove that the 50v warning light will work with just 50v so using a simple battery may in fact be better? The better proving units step up through the range of voltages.

My father-in-law had kittens when he found a worker who had been told all cables must be spiked before removal with a band-jack spike about to knock it through the cable with a sledge hammer. This points out how careful one has to be that ones instructions are fully understood.

The neon voltage tester to my mind is the tool to test for dead with. It will because of the lead attached likely show live even if the wandering lead is not connected to a neutral or an earth. In other words just like a neon screwdriver. I used the neon screwdriver to test for dead as well as a proper tester when working on an IT system. It was so hard to find something which was reliable to test to. Clearly as not an earthed system earth was no good so one had to find some other item powered from same transformer.

Try testing a shaver point for dead without connecting to both live wires. Can't be done. One has to open it up before one can test for dead. A faulty switch can really be a problem.

So it is often a case of treating it as being live even though it should be dead.

 

[JohnW2]

Mine doesn't...

That's interesting. I wonder if that's related to the fact that it's an autoranging meter? Mine gave lower readings on lower voltage ranges - but I showed you the result on the 600V range, since that's the one which would be used for checking 230V AC. A more realsitic test, in context, would be to use a much larger DC voltage. If I get time, I'll try that tomorrow - but it would be even more interesting with yours, since it's autoranging.

Kind Regards, John.

 

[securespark]

Tut, tut, Rob....Where's your green edition?

 

[Spark123]

A Multimeter set to AC volts will only register AC volts, not DC. If anything all you would register is the ripple (which is handy for seeing if your capacitors in a PSU are failing).
Similar vice versa, a DC instrument wont measure AC.

So when was the last time you got DC on a AC circuit?
But I have seen a lot, AC induction on DC.

I come across distributed emergency lighting systems which operate on both AC and DC, AC for the maintained side of them and DC when on battery. So the answer is two days ago.
I use a fluke 73 and a fluke 77, both do what I said above - they don't give any meaningful reading when measuring AC when set to DC or vice versa

 

[dormermike]

Really interesting discussion!

I always use a MM for testing circuits prior to working on them but can see now the potential dangers. As a belt and braces sort of chap i always first switch the circuit on or plug a lamp into socket and make sure lamp is lit. I then turn off circuit at consumer unit and observe light no longer illuminated. I then check the MM on correct setting testing both live and neutral against the bundled earth, then if all combinations show zero i then check against central heating pipework or similar. Finally before touching anything with bare hands i 'short' all the wires using a screwdriver which has a rubber handle. No sparks, good to go.

Should be sufficient but always open to sugfestions on how to work more safely.

 

[Alarm]

I come across distributed emergency lighting systems which operate on both AC and DC, AC for the maintained side of them and DC when on battery. So the answer is two days ago.
I use a fluke 73 and a fluke 77, both do what I said above - they don't give any meaningful reading when measuring AC when set to DC or vice versa

But in that instance you would expect to see both. So test for both, easy really.

My comment was about the standard AC circuit being tested. Anything with backups in place should be tested for both IMHO.

 

[JohnW2]

So when was the last time you got DC on a AC circuit?

I come across distributed emergency lighting systems which operate on both AC and DC, AC for the maintained side of them and DC when on battery. So the answer is two days ago.

As Alarm has responded, that's rather moved the goalposts. When we asked virtually the same questions, both he and I asked about 'an AC circuit' (and I added 'domestic'). As he said, if one knows that the circuit may carry DC, one has to formally satisfy oneself that it is dead in relation to both AC and DC.

I use a fluke 73 and a fluke 77, both do what I said above - they don't give any meaningful reading when measuring AC when set to DC or vice versa

That's interesting. I guess I need to withdraw my previous 'rubbish' comment and substitute some apologies! However, it remains the case that I have never seen a multimeter which behaves as is being described. Needless to say, none give meaningful (if any) readings when measuring AC on a DC range - but every one I can remember has given readings (of varying degrees of 'meaningfulness') when measuring DC on an AC range. I must have a dozen or more multimeters scatterd around the place if I can find them, so I'll undertake a 'survey' when I have some time.

Mechanistically, it's quite hard to imagine how/why those AC voltmeter which don't respond to DC don't respond to it. The most obvious explanation would be a capacitor in the way, but I rather doubt that, unless it's done deliberately to prevent it responding to DC. If I find that any of mine don't give readings with DC, I'll get out my screwdriver and see if I can determine the reason!

Kind Regards, John.

 

[Spark123]

I come across distributed emergency lighting systems which operate on both AC and DC, AC for the maintained side of them and DC when on battery. So the answer is two days ago.
I use a fluke 73 and a fluke 77, both do what I said above - they don't give any meaningful reading when measuring AC when set to DC or vice versa

But in that instance you would expect to see both. So test for both, easy really.

My comment was about the standard AC circuit being tested. Anything with backups in place should be tested for both IMHO.

Some motor drives have AC and DC too which I am sure someone mentioned before.
Why test for both AC and DC on a circuit when I can use a proper voltage indicator which will tell me there is voltage present without setting any ranges etc.
An approved voltage indicator (to GS38) used correctly is far less likely to bite you than a multimeter is.

 

[Alarm]

As before, I have both. Depending where I left the damn things I can use either with confidence.
The point we are making is it does not specifically say you cannot use a MM.

Your work regs might do so, we have covered this.

I reiterate, either can be used in a safe manor to come to the end result. The two probe tester being the quicker way.

 

[Spark123]

GS-38 specifies two types of voltage detection instruments - neither of which are a multimeter.
See part 12 here: http://www.hse.gov.uk/pubns/priced/gs38.pdf
It depends on whether or not you are testing for voltage (voltage detection) as you would in an isolation, or carrying out a voltage measurement (which you may use a multimeter for - commissioning etc), see part 14.

 

[Alarm]

Seen it, read it.

Whats this then?
12
(a) detectors which rely on an illuminated bulb (test lamp) or a meter scale (test meter). Test lamps fitted with glass bulbs should not give rise to danger if the bulb is broken.

Test Meter? Would that be a Multi Meter?

Not trying to argue here, but thats from your link.
As it was the last time it was posted ( Not sure if it was you, just saying).

 

[Spark123]

No - it is not talking about a multimeter. The instruments they are talking about are "solely" used for detecting voltage. A multimeter does not solely detect voltage.