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Topic Title: Calibration of hand held electrical instruments
Topic Summary: The need to maintain calibration for service engineers instruments.
Created On: 27 September 2018 03:10 PM
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 30 September 2018 10:34 PM
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sparkingchip

Posts: 11449
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I have an ex-MoD AVO, which is really a bit of a novelty item I bought off EBay that does not get any serious usage.

I did a job some years ago for a technical guy from a motor control-gear manufacturer, he kept one in the boot of his car when working as he said it was a good problem solver.

I'd be very wary of doing much with it on live circuits, I have had a few mini disasters such as blowing up a tone generator by forgetting to disconnect it before reenergising the circuit. The expression "Sh*t happens" comes to mind, it's hard trying to be perfect all the time.

Andy B.
 01 October 2018 09:34 AM
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mapj1

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Avo meters are quite fun, and are genuinely useful when you have a lot of RF around, as unlike the microprocessor marvels you cannot crash the processor in a high electric field. But not I think a suitable field instrument for the busy mobile technician - the odd one per lab is enough. The size and weight, and the scope for selecting the wrong range and thonking the needle into the end stops (which is how the springs get strained) is enough to make them 'handle with care' .
lower pic here shows the sort of thing

Modern resistor chains for shunts and so forth will not be open wire wound like days of yore, and will not corrode.
These will not drift appreciably over the life time of the equipment, or indeed the lifetime of it's owner, we can, and do, design electronic sensing equipment for better than 1% over a 100,000 hour operational life as a matter of routine.

So what you are really doing is verifying performance, to see if things have been subjected to damge - if the current range is overloaded so the hermetic encapsulation on the shunt resistor is cracked or in extremeis reduced to a sooty mess, then ideally the component should be replaced, rather than the instrument gain adjusted to compensate, as it will never be the same again.

In general you might expect the internal parts that are indended do the amps and ohms ranges to be the ones that don't appreciate connexion to 415V or whatever, and these are the things that need checking more often to see if there have been any 'events'.
.

Actually the first line of diagnostic can often be the mk1 human nose, there is an old joke about all electronic designs being built with a small amount of magic smoke encapulated that makes it work ,and when this is released, the magic is gone and it will not work again.

And indeed, to all of us, one day something like that will happen, and this is far more likley with the testing of faulty things while under time pressure.

-------------------------
regards Mike
 01 October 2018 11:27 AM
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sparkingchip

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I now often use a screwdriver blade to press RCD test buttons or, as Alan Capon recommends, use the RCD tester rather than pressing the RCD test button, after I pressed a button with my finger and the RCD blew up.

Andy B.
 01 October 2018 11:31 AM
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Zoomup

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I can envisage that the new "all safe" MIT400 range will get new users into bad habits. "I just can not damage this test instrument because it is inherently safe", they will say. They will give up thinking. They will go into auto-pilot mode. So when exact connection and button pushing IS required elsewhere on other instruments we will see a big bang and flash and blackened faces or worse. It is a bit like driving an automatic gearbox car where you just can not select a wrong gear, then you go to drive a manual gearbox car where thinking is required. After some years of driving automatics you will lose the ability to safely drive manual gearbox cars and select the correct gear for use.

Z.
 01 October 2018 01:21 PM
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sparkingchip

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I have a van and a pickup, the indicator lever is on opposite sides of the steering columns in them, you learn not to indicate with the windscreen wipers as you see them go across the windscreen in front of you.

Whilst on holiday in Devon this year we gave some Dutch hikers a lift back to Lynton, apparently they came down Countisbury in a camper van with an automatic gearbox unaware that you could change into manual rather than standing on the brake pedal and arriving at the bottom with smoke coming out from under the camper van. At least the brakes worked and there are escape lanes on the way down the hill.

The "foolproof" auto sensing multimeters like the Kewtech only have two test lead sockets and don't measure current, so you cannot wrongly attach the leads in the first place. Other quality multimeters are designed to fail safe if inadvertently misconnected.

What you don't want is something that will fail totally if misused and cause serious injury or worse.

Edited: 01 October 2018 at 01:46 PM by sparkingchip
 01 October 2018 01:35 PM
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chrispearson

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Originally posted by: intrinsic4225B

Don't forget that the moving coil meter is only one part of the measurement circuit - before the moving coil meter you have resistors or resistor chains functioning as dividers and shunts, all of which can be subject to drift in their values over time.


Thank you - I'd forgotten them.

Originally posted by: mapj1

In general you might expect the internal parts that are indended do the amps and ohms ranges to be the ones that don't appreciate connexion to 415V or whatever, and these are the things that need checking more often to see if there have been any 'events'.


I still find the AVO useful for automotive work. With dodgy connexions, it is useful to see not just how far the needle moves, but also how it moves.
 01 October 2018 01:48 PM
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sparkingchip

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 01 October 2018 02:55 PM
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Zoomup

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I would not be happy using that test instrument on a 12 Volt car system, let alone "mains".

Z.
 01 October 2018 03:28 PM
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mapj1

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In a workbench repair type setting, downstream of the fuse in the plug feeding the bench, I 'd be happy to allow it, not though if more than one phase is involved.
Despite the low price tag, it is after all Cat II 600V, which means it has been tested to survive surges of 4000V DC.
That said, on the bench beside me is a hand held Tektronix of a kind not made since about the year 2000, that I think then went on to inspire some of the later fluke designs when they got taken over, and it is Cat III. I am less sure how the Lap model will perform in 20 years time.

The biggest concern to me with that model woulld be the sort of accident possible with the non-fused 10A current range, and from a 'fumble fingers' point of view the fact the fused 200mA current range is the same terminal sockets as the voltage range, and the 10A is not.
And as a reminder

Category I -- For connection to circuits in which measures are taken to limit transient over-voltages to an appropriately low level.

Examples: Protected electronic circuits.

Category II -- Energy-consuming equipment to be supplied from the fixed installation.

Examples: Appliances, portable tools, and other household and similar loads. Measurement equipment intended to measure the voltage levels of these loads must be rated at this overvoltage category.

Category III -- In fixed installations and for cases where the reliability and the availability of the equipment is subject to special requirements.

Examples: Switches in fixed installation and equipment for industrial use with permanent connection to the fixed installation; measurement equipment intended to measure the voltage levels of these fixed installations must be rated at this overvoltage category.

Category IV -- Used at the origin of the installation.





see here for a nice summary by national instruments

-------------------------
regards Mike
 01 October 2018 05:07 PM
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KFH

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I find that most of my diagnostic work on fixed wiring and some appliances is done with my Fluke T150 voltage tester. No ranges to worry about no need to worry about blowing it up on 440V, voltage readout and LED voltage range. Live indication, phase rotation. DC/AC display, continuity buzzer and as a bonus press a button and it will read Ohms before automatically defaulting to voltage testing. If you do not need to test current it is an excellent tool.

I would not rely on the Cat of cheap instruments there are too many examples on Youtube of the ways they do not comply.
 03 October 2018 04:46 PM
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sparkingchip

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I have just used the Megger 420/2 to test a commercial pressure washer by insulation testing across the plug pins, along with the insulation test result it displays the voltage across the test leads whilst the test was carried out. The initial test result was 17 Kohms with 23 volts maintained across the leads, having cut the dodgy looking plug off the flex and fitting a new one it then returned a tests results of 12 Gohms with 507 volts maintained across the test leads.

I am not sure I need to confirm the test voltage, but it does give that extra reassurance that things aren't or are as they should be.

Andy B.
 07 October 2018 09:53 PM
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Jaymac

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"Intrinsic 4225B" makes as good point,
Are you only checking the resistance ranges of the instruments, or do you have the means to check the voltage ranges (e.g. against a calibrated multimeter kept solely as a reference)?
The short answer is no,
Checks are made against a Seaward check box. It has the following:
Resistors, - 1?, 5 ?, 10 ? and 1 M?. The resistances are used to check the continuity ranges on insulation testers and the I M? resistor used to check the insulation ranges on the insulation testers.
A 13amp socket and a voltage standard which is 75.4 volts. The facility is seldom if ever used as the accuracy of voltage indications given on a digital voltmeter is/are generally quite stable. (I hasten to add that this does not mean that the voltage reading can be treated as unimportant as it is first and foremost when properly conducting a "safe isolation" procedure).
I had a discussion with a colleague - about doing accuracy checks and composition of check sheets on electrical measuring instruments. His opinion is that anyone undertaking checks or composing check sheets should have a recognised qualification to do so, regardless of how well experienced the person might be. He cited a situation where an accident might occur would result in all documentation being brought under inspection and the signature of anyone who did not have the proper accreditation would be worthless.
Is the GS38 proving unit suitable as an indication that a volt meter is working ok. This begs another question - what is the suggested interval between checks on a proving unit.
 07 October 2018 10:56 PM
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mapj1

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the signature of anyone who did not have the proper accreditation would be worthless.

Not quite, some one who could not show they were competent, may be in trouble, but only of course if there was a clear link to the accident and the passing of defective test equipment as fit for service - that would be the first hurdle in many cases.
Then, there would be trouble for the tester, regardless of any gold plated paper trail and collection of certificates. And actually, without said paper trail, competence can still be proved by historic behaviour, but I agree can be a pain.

I think your friend is over thinking it. In the case of checking the ohms range on a meter against a known good load, and perhaps testing the known good load against the one meter in the company that gets sent for cal when it has just been done, the risks are very low.

The only credible mortal peril may be if a meter on the volts range is being used to decide if something is safe to touch- and any half-decent procedure says check the indicator on a known live circuit, before relying on it so show something you wish to touch is dead, for exactly this sort of reason. If the Zs or R1 & R2 is measured wrong, unless its out by an enormous factor, all that happens is that the 5 second breaking time may be 8 or 2, and often the DNO can introduce that sort of change for free, with no announcement.

-------------------------
regards Mike
 07 October 2018 11:06 PM
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intrinsic4225B

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Jaymac,

Probably the first thing you need to do is to establish why you are doing the checks and what you want to acheive from them?

Do you operate an ISO 9001 quality management system or similar? If so the requirements for the control of monitoring and measuring resources should be documented within that system.

If you are doing it for compliance with the requirements of a accrediting body or scheme provider, then ask them what they expect from the system.

In terms of qualifications for the person carrying out the checks - it would be interesitng to ask you colleague what they would consider a recognised qualification for carrying out basic checks on instruments as it is not something that you are likely to find a specific qualification for at your local college.

In reaility it is something that could reasonably be carried out by a competent electrician or technician who has a understanding of what is intended to be acheived. See EAWR 1989, Regulation 16 and HSE GS38 Paragraph 30.

I recall correctly the voltage output of a Seaward 16 checkbox is not a fixed voltage, but varies with both input mains voltage and with loading - this is the kind of additional knowledge that someone carrying out instrument checks would need to know.

It should be noted that it used to be the position of some accrediting bodies / scheme providers that they would expect checkboxes only to be used for intermediate checks of instruments between calibrations by the manufacturer or calibration laboratory.

In terms of the frequency of checks - there are two strands to this:

If the instrument is being used for proving dead, then it should be checked for correct operation immediately before and immediately after it has been used to prove the conductors dead. See HSE GS38, Paragraph 20.

Other instruments that are being checked for accuracy of indication should be checked at an interval determined based on the critcality of the measurements they make and the enviroment in which they are used (i.e. how roughly they get treated!).

It should be noted that the use of multimeters for proving dead is discouraged by the HSE, see HSE GS38 Paragraph 19.

Although everyone will say they know how to use a multimeter properly - its not so much a matter of knowing how to use it as not making inadvertent mistakes, perhaps particularly when under pressure and trying to get a machine comissioned or back into service!

We all probably know how to drive a car - but we've all at sometime tried to move off from from the traffic signals with the handbrake still on!
 09 October 2018 09:49 PM
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sparkingchip

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 09 October 2018 09:58 PM
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sparkingchip

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Okay that was the short video.

Last night my wife came home from book club to find me asleep on the settee with thisHour long video playing on the TV.

I resumed watching it, but I'm not sure what I missed as I don't know how long I was asleep.

So your challenge, should you wish to accept it, is to watch the full video and comment on it. Though some of you like Mike might like to skip through to the highlights. There is an excerpt included from another YouTube video I have watched in which the guy ripped the finger contact pad off the back of one, apparently to make it safer.

Andy B.
 09 October 2018 09:58 PM
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mapj1

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now that is a neat toy. presumably AC only but still that is 90% of the time.

-------------------------
regards Mike
 10 October 2018 08:29 PM
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sparkingchip

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So would you be happy keeping your finger on the touch pad?

Unlike a neon screwdriver you are not actually part of the circuit, are you.

Andy B.
 11 October 2018 12:27 AM
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mapj1

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yes, exactly as you are with a voltstick, a neon or an other, your body is a loose capacitance to earth and completes the circuit. BUT the impedance is far higher than the neon lap that needs a decent fraction of a mA to strike, and here we are looking at microamps though the body. Perfectly safe.

-------------------------
regards Mike
 08 November 2018 09:31 PM
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sparkingchip

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I went to see what two PV installers were up to this afternoon, one of them has a Fluke T6-1000 and let me have a play with it.

He said he likes it as it measures DC, so is ideal for the work they do.

I enquired if it would work with me standing on some steps, but ended up kneeling on a chest deep freeze. Using the Fieldsense it displayed the amperage on a meter tail, but the voltage was way off at 56 volts. I started looking for some to put my finger or the negative test probe on, but there was nothing within reach. The PV guy said it dies give some odd voltage readings using the Fieldsense, but it didn't seem to worry him or put him off using the tester. Most of the time you can get the test probes onto terminals anyway.

It seems like a nice bit of kit that will be okay in experienced hands.

Andy B.
IET » Wiring and the regulations » Calibration of hand held electrical instruments

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