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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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 27 September 2018 03:10 PM
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Jaymac

Posts: 38
Joined: 24 September 2009

I read with interest some of the posts from a topic on calibration which was put on some time ago.

I would like to put a question regarding the requirements for those employed, not as installation electricians, but as engineers whose job will involve the electrical connection of mechanical equipment such as pumps gear boxes and conveyors etc.

In some instances the engineer involved will need to check the availability and type of supply - whether single or 3 phase - the voltage level of live connection to a control panel, remote control station or joint box adjacent to the equipment to be fitted.

The said engineer would have adequate training and electrical awareness for the operation in hand although not qualified as a time served electrician. Is there guidance on how a multi skilled engineer should be qualified?

What would be the current recommended requirement for instruments this engineer might use, i e multi-meter, volt meter, ammeter etc.

Should they be category 3 and calibrated annually?

Multi-meters with no category marking can be bought in hardware stores as can clamp-meters for a lot less than those available from proprietary electrical supplier.

Any I compared with the more expensive ones were "reasonably" accurate although the quality of the test leads were thinner and the build quality not as good.


I would be grateful to any of you enlightened people who would give an opinion on the above comments.

Regards
jaymac




Not to be confused with Jaymack who see posts quite often.
 27 September 2018 05:01 PM
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mapj1

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I suggest that the calibre of test gear used should depend on the penalty of a mis-reading, and the risk posed by mistake or malfunction.

So if it is used for maintanance like checking a replacement heater or similar, just looking to see if some current is drawn or none, and the current drawn is more or less the same on all 3 phases, then a non-calibrated clamp meter is probably just fine.
It only really needs to be re-verified if it gets dropped down the stairs or the mains gets applied to the amps range and somerthing goes bang.

If however it is to be used to decide pass/ fail limits where systems are expected ot be within a few percent of the boundary, and failure implies expensive re-works (as opposed to first calling for a second opinion) then the accuracy spec required warants annual calibration, certificates to show it etc.

IF it is used to make life or death decisions - is it safe to usncrew the lid now ? then again some regime of checking, but ideally one that is 'self check' so it can be done there and then is in order.


You can probably remove a great deal of the need for everything to be calibrated, by sensible procedures for problem escalation and to do for things that look funny.

For things like the lower category of kit or thinner test leads, I'd be asking is this meter always used on lightly fused low energy supplies - if there is the intention it will be used to measure something capable of vaporising said lead, it may be false economy. (but if the intention is only to use to the ohms range on dead things, then it does not matter so much)
Fused leads and probes can usually be retrofitted however.

-------------------------
regards Mike


Edited: 27 September 2018 at 09:28 PM by mapj1
 27 September 2018 05:45 PM
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sparkingchip

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I recently bought a Megger MIT420/2 Cat IV Insulation Resistance Tester and everything about it is far superior to my Megger MIT320 Cat lll insulation tester, particularly the test leads.

In particular the test lead connections are fully shrouded, you cannot pull the clips off and shove the connector in to use it to test with or without a fiddly little bit of plastic that is supposed to shroud the connector to comply with GS38, you have to attach the correct probes or clips. I did have a mishap with a set of the Cat lll leads once using without the fiddly little bits of plastic.

Admittedly I don't actually know how to use all it's functions as yet and still need to get up to speed with it.

It came with a calibration certificate and I'll do self checking to spin the calibration period out as long as possible, probably three years.

I would not want to be using the type of testers you are suggesting, all my installation testers are rated at least Cat lll and checked reasonably regularly.

Andy B.
 28 September 2018 09:53 AM
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Jaymack

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Originally posted by: Jaymac
The said engineer would have adequate training and electrical awareness for the operation in hand although not qualified as a time served electrician. Is there guidance on how a multi skilled engineer should be qualified?

Using electrical test instruments is hazardous for any person although the fault level in the domestic are usually scary but not injurious. I use one in the shed, but I have to discipline myself to leave a multimeter on the highest voltage scale after use for resistance tests etc. I think these are particularly dangerous if used on higher power levels in the industrial scene, since there are no fused leads or inbuilt protection other than a suspect fuse.

I remember a trial in combining disciplines in a new aluminium smelter where I was employed as an electrical engineer, when the only discipline employed was millwright, albeit all the applicants were electricians and fitters; this was an attempt to have only one common union for the engineering team due to the strategic nature of aluminium smelting, (subsequently scuppered by the other unions). The company in their wisdom issued small, cheap multimeters to all the "millwrights" ...... this was quickly stopped after some scary incidents.

In short, I would be wary of asking a non electrical person (or any electrical person) to carry out electrical tests, unless they were adequately trained or frequently exposed to a particular routine.

Regards
 28 September 2018 10:53 AM
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Legh

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IMO with a good quality mulit-meter such as a auto-ranging Fluke, Always turn it off after use as this will preserve the battery and you'll be unlikely to use it on the wrong voltage, current or resistance on start-up unless you're a dingbat.....

Also i would suggest investing in a mains Voltage operated Portable Appliance Tester. as the battery operated ones are unable to meaure accurately wattages provide high current earth testing, Makes a good paperweight too.....

Legh

-------------------------

http://www.leghrichardson.co.uk

de-avatared
 28 September 2018 08:24 PM
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sparkingchip

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I have a Kewtech auto multimeter which is Cat lll and supposedly idiot proof.

I would not describe it as fully idiot proof, as if you put the probes on two terminals both at the same voltage it will show the resistance between them rather than warning that there is a voltage present, as will any test instrument that will automatically switch between measuring voltage and continuity.

Andy B.
 28 September 2018 08:29 PM
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sparkingchip

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And that cat lll tester is not an expensive instrument.
 28 September 2018 11:19 PM
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chrispearson

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Originally posted by: sparkingchip

... if you put the probes on two terminals both at the same voltage it will show the resistance between them rather than warning that there is a voltage present ...


How could a meter possibly measure the voltage between the terminals and the reference point, by which I assume you mean earth?

I do, however, get the gist of your point. Did old fashioned AVOmeters ever go out of calibration? Presumably not - rather like, say a mercury sphygmomanometer, their calibration was based upon a physical constant.
 29 September 2018 08:16 AM
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sparkingchip

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If you measure the voltage across the live and neutral terminals with any voltage indicating tester of say an electric heater,, even a simple lamp, ,and the neutral is disconnected in the wall switch, both the terminals will be at the same voltage and the tester won't show any voltage.

Using a tester such as the Kewtech smart multimeter it will display the resistance of the heater and not show a voltage is present, you won't blow it up by having set to measure resistance when there is a voltage present, but it's not fool proof as I was told it is, because you need to know what you are measuring.
 29 September 2018 08:17 AM
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sparkingchip

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 29 September 2018 08:18 AM
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sparkingchip

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 30 September 2018 10:56 AM
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sparkingchip

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Originally posted by: Jaymac

Should they be category 3 and calibrated annually?

Multi-meters with no category marking can be bought in hardware stores as can clamp-meters for a lot less than those available from proprietary electrical supplier.

Any I compared with the more expensive ones were "reasonably" accurate although the quality of the test leads were thinner and the build quality not as good.

I would be grateful to any of you enlightened people who would give an opinion on the above comments.

Regards

jaymac

Not to be confused with Jaymack who see posts quite often.


I have just read the Fluke multimeter guidelines again.

It explains why your proposal to buy cheap testers could result in someone dying or having been seriously injured having drawn an arc as a result of using that cheap multimeter or some other issue such as having the test leads inserted into the wrong sockets.

Andy Betteridge
 30 September 2018 04:43 PM
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sparkingchip

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 30 September 2018 07:47 PM
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mapj1

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The old moving coil instruments needed calibration as the little spiral hair springs that returned the meter needle to its zero position could easily be strained, and then the deflection per milli-amp or whatever would move.
There is precious little to calibrate in a modern instrument, even the little twiddly resistors of 20 years ago that set the full scale value have now given way to constants in a flash memory of 'calibration constants' which are really the magic numbers the software multiplies the ADC reading by to decide what value to display.
The thing that seems to need updating more and more frequently is the software version, as test gear internally 'boots' the equivalent of windows for test meters.
If you don't throw it down the stairs or store it somewhere damp, and you don't stuff mains up it on the ohms range, then a modern instrument will not 'go out of cal' pretty much for ever.
As those of us who carry check loads instead of periodic re-cal will confirm, they only misread after you have done something silly or damaged it, or both..
A cynic might think the new models go on sale before the software is fully 'debugged'.

-------------------------
regards Mike
 30 September 2018 08:11 PM
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chrispearson

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Originally posted by: mapj1

The old moving coil instruments needed calibration as the little spiral hair springs that returned the meter needle to its zero position could easily be strained, and then the deflection per milli-amp or whatever would move.

There is precious little to calibrate in a modern instrument ...


Mike, thank you. So instead of taking my MFT back to Megger, I should take my Avometer Eight Mk 6?

I can see how the zero point might change, but you'd see that; however, I cannot quite see how the spring constant would change, or if it did, how recalibration could correct it.

And what of the little spiral hairspring in my Rolex does that go out of calibration?
 30 September 2018 09:10 PM
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sparkingchip

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What price to have a watch serviced, cleaned and a new battery fitted.
 30 September 2018 09:54 PM
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Jaymac

Posts: 38
Joined: 24 September 2009

Thanks to all of you for taking the time to reply.
It is good to get informed opinion on what is right from other people in the electrical business, other than those who are sales orientated.
I have some responsibility for the procurement, maintenance and condition of instruments for a small workforce.
I keep a register with each instrument documented and with the aid of a check box with high stability resistors, a check was/is made, the result noted and assuming it gave a reading within +/- 5% of that which it was compared to it is considered OK. The check box is calibrated annually, and the accuracy of the instruments checked annually. As a result, I need to look at how I do things and try to "keep ahead of the posse" so to speak. It has been suggested that a recognised qualification from a suitable training organisation is needed for the person keeping these documents or alternatively, have it done by an accredited calibration company.
As time moves on and Health & Safety directives become more demanding, and customers and consultants acting on behalf of customers, sometimes ask for documentation as proof of a recognised level of management before a tender for a contract can be offered or accepted. This includes pat testing of all appliances, certification of loler inspection for lifting chains and slings and so on.
"Sparkingchip" in his reference to the HSE GS38 document made a very good point - I read it and found that it contains explanation on what the law requires along with lots of other stuff. The Martindale guide brochure clarifies where the boundary's lie as regards the category of instrument to be used. I looked at a Fluke guide which shows something similar.
Our company put everyone through a course on "safe isolation". The instrument companies offer a kit with a proving box for this purpose.
It seems that the minimum requirement for work on mains voltage installations, CAT 3 and for work in or near the main distribution board it work should be CAT 4.
I had a look at the Megger MIT420 video - It looks like the real deal - costly but what price safety.
And guess what, we still have an AVO - I think it is a Mk8 .
Regards.
Jaymac.
 30 September 2018 09:59 PM
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intrinsic4225B

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Originally posted by: Jaymac

In some instances the engineer involved will need to check the availability and type of supply - whether single or 3 phase - the voltage level of live connection to a control panel, remote control station or joint box adjacent to the equipment to be fitted.


In this sort of situation, if all that is required is an approximate voltage measurement, a simple voltage indicator may be more appropriate and more cost effective.

A example of this type of instrument would be the Fluke T90 Voltage Tester which indicates voltage levels of 12V / 24V / 50V / 120V / 230 / 400V / 690V DC or AC and has an overvoltage category of CAT III 600V.

I have no connection with Fluke other than as a satisfied user.
 30 September 2018 10:11 PM
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intrinsic4225B

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Originally posted by: chrispearson

I can see how the zero point might change, but you'd see that; however, I cannot quite see how the spring constant would change, or if it did, how recalibration could correct it.


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.

Its worth noting that the internationally accepted definition of calibration is that of comparing the indications of a measuring instrument with a measurement standard (i.e. a known value) and does not include adjustment.

Strictly an measuring instrument must first be calibrated to determine if adjustment is required, then adjusted if necessary, then calibrated to ensure the adjustment has been successful!
 30 September 2018 10:17 PM
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intrinsic4225B

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Originally posted by: Jaymac

I keep a register with each instrument documented and with the aid of a check box with high stability resistors, a check was/is made, the result noted and assuming it gave a reading within +/- 5% of that which it was compared to it is considered OK.


Are you only checking the resistance ranges of the insturments, or do you have the means to check the voltage ranges (e.g. against a calibrated multimeter kept solely as a reference)?

If the main use of the instruments is the measurement of voltage, then only checking the resistance ranges could allow for criticism by an auditor or other interested party.
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