Test Meter Recommendation

[Tim Streater]

[screen loads of empty shit snipped]

Could ye learn to snip d'ye think?

[John Rumm]

If they also stick an external standards body stamp on the case, then IIUC they are also supposed to submit the device to said body for proper testing.

Those photos do not really inspire any confidence it would actually perform up to the claimed standards.

[John Rumm]

Yup, I doubt Maplin do much if any evaluation of the actual technical specs.

I actually agree with you on the odds - most of the time.

It does help to have a feeling for how "stiff" the supply is in any given circumstance though. And there is one of the chicken and egg situations, where having proper test equipment that you can plug in and get a quick indication of the supply impedance and PFC from the off is rather useful.

Secondly, even if you are only moderately keen on DIY there is a good argument for having adequate test gear to be able to carry out a full range of tests, something that a multimeter is not going to let you do. £100 - £150 on ebay will most likely get you some professional grade kit second hand which will not only let you do all the tests, but also in the knowledge that its likely to fare better if things go pear shaped as a result of events beyond your control.

[dom]

Ok, I've just had another look at my UNI-T UT526.

The manual says "Designed and manufactured strictly in accordance with IEC61010 safety standard, the instrument meets with Over Voltage Standard (CATIII600V)..."

And there's no approvals marked on the instrument itself (other than CE which is self-certified)

So it's all self-certified.

[John Rumm]

And neither should it. As always, you size upon the less salient point and ignore the important bit. I presume this is by intention since you feel the need to "win" the argument?

The key factor in reducing PSCC, is the length of 2.5mm^2 cable (or whatever) between the point of contact and the socket. Even if you are on a very stiff supply, by the time you are 20m down a circuit, or inside an appliance on the end of a plug and flex you have the comfort of knowing that there will be 500A or less of fault current to play with regardless of what is available at the incomer.

Who mentioned the meter itself introducing a spike?

The spikes come from a lightening strikes, or motor switching, inductive back EMFs, or your neighbour welding something etc. i.e. not things you have much control over.

Will your meter maintain that 10MOhm impedance in the presence of an

8000V transient as required in the CAT III tests? Are the track spacings adequate to not suffer breakdown? (they may well be, have you looked inside at the construction?)

Of course. A DMM at any price range can't even do all the required tests anyway.

So how do you do an insulation resistance test with it?

A RCD ramp test?

How do you time the RCD trip time?

A Non tripping ELI measurement?

How about accurate low ohms measurements of R1+R2 loops etc?

Did anyone claim otherwise? You need the prerequisite knowledge to use any test equipment safely. Hopefully that would also include the knowledge of the limitations of that equipment.

Flights of fancy? These would be the same flights of fancy that have killed people, and also result in injuries each year I take it?

I can't see much scope for measurement of current during DIY wiring work. If you do need to measure mains current draw then you are much better off with a clamp meter in the first place.

However I bet even your entry level Fluke will contain at least some input protection, have shielded compartments round the probe sockets and MOVs on the board etc, so if you accidentally stick it on mA or Ohms and then connect it across your main incomer terminals, its less likely to blow up in your face. The moral here is you get what you pay for,

Two final notes?

Not generally. However looking at the logs from a UPS can be quite enlightening. I have seen some (in a domestic, rural installation) where transients in excess of 1kV occur several times every day, along with other disruptions such as sags and brownouts. (the UPS was installed after transients killed pretty much every major component in their computer)

[orion.osiris]

I do apologise. It's Google's fault but I'm working on it.

[Archibald]

All the above are beyond the scope of normal DIY work. The original post referred to a Megger and earth testing. Both these tests can be completed with a DMM and a few additional "off the shelf" items.

For DIY work I would think a DMM is the best approach. Personally I have never felt the need to do an insulation check using high voltage anyway. I tend to trust the cable manufacturer and then there is a breaker that would trip. Earth loop impedance wouldn't be a problem either.

RCD testing is taking things to ridiculous levels. If I had access to the equipment to test the things, I wouldn't bother anyway. I couldn't certify the tests so there wouldn't be a lot of point doing more than a button press.

Read the "operator error" bit.

I dont recollect ever needing to check incomer Volts. A quick jab on the RCD test button saves getting the meter out.

Yep Interruptions I'm afraid. No rest for the wicked!

Domestic rural! Long cable runs and a few farmers adding their unique "footprints" to the supply, not too surprising I suppose.

I would have thought a PC power supply was fairly immune though, lots of ferrite in the thing and a fully wound transformer to produce the very much lower Voltage. I suppose there can be more than the one obvious route into the thing though.

AB

[Frank Erskine]

Which rather suggests that consumer units etc. are outside the scope of DIY... (be careful here!)

Only if the user is aware of possibly misleading indications of a high input-impedance meter :-)

Except under fault conditions, which is exactly what you're looking for...

[Archibald]

They are at the input side. Anyone disconnecting the device or working on the input would have a live set of meter tails to deal with.

Diagnosis of problems within the unit is straightforward and wouldn't normally call for test equipment anyway.

Let the user beware! Avo sevens went out the window years back, so very few meters now present much of a load. Seriously though I got caught out on this one myself using a two cable LED "prodder" bought from CEF. I stuck the thing on a known "dead" cable just to be sure that I could get stuck in with the screwdriver and to my surprise it was live. After stripping back the trunking and cursing the moron that had numbered the cables I found it was simply open circuit at both ends. Capacative pick up from an adjacent cable had lit the appropriate number of LEDs up.

I never associated a "prodder" with sensitivity, but now I know!

A second little snag [reading 100V on a 110V supply derived from the

55V and center tap of a transformer] saw the thing consigned to the bin! My fault, the next LED up from 50V was 100V, this wiring error was a first for me, so I wasted a lot of time looking for obscure instrumentation problems! The electrician must have had an off day on that project as we had turned out hundreds with the correct supply!

The most common fault is a tripping breaker supplying a heating element. The most simple diagnostic tool is the breaker itself. Disconnection removes the fault and stops the tripping. Even with a Megger disconnection of the element would have to be carried out at some point.

More obscure faults can also be found in the same fashion, just disconnect until the tripping stops. Then mop up the puddle!

Earth loop impedance would need a little more than a DMM if you didn't want to superglue the breakers :-), but I have more than enough confidence in a few more basic checks to ensure that my earting is safe and the RCD picks up the problem.

AB

[John Rumm]

For the complete bodge artist, perhaps. For those who want to see jobs done to a proper standard, they are certainly not beyond the scope.

Off the shelf 500V sources? What did you have in mind, a microwave oven?

That is an enlightening paragraph. Not sure there is much I can say really.

RCD testing is a *requirement* of the wiring regs. If you are installing one, or checking the operation of one, you need the right test gear. RCDs can and do fail.

Have you not read the multitude of nuisance trip posts on this group?

Do you not suppose having the kit to test the thing might actually prove rather useful to the DIYer? It will cost less than getting an electrician in to play hunt the fault.

Yea sorry, I was coming at this from the engineering perspective that the purpose of testing is to make sure that the stuff actually works correctly and is safe, not from the pointy capped perspective of making sure all the paperwork lines up.

Your standards may well be different to mine.

A moment ago you said you could do ELI checks with a DMM. How were you proposing to do those without also measuring the supply voltage?

Indeed - it had suffer several serial failures of different bits, before on one occasion suffering a dead PSU, mobo, and optical drive in one hit. Fortunately the only surviving bits included the HDD (which I cloned and replaced anyway).

[Archibald]

Rubbish, get real! I doubt even a lot of professionals would test at this level if they didn't have to certify their work.

Oh dear! we're not too good at lateral thinking are we?

Er there is a test button, or hadn't you noticed?

Yes, are you suggesting they all rush out and buy the kit to test the things? Get with it!

Er let me see, what does an RCD cost, how long to replace? I would imagine the electricians first visit would involve a brief appraisal of the load, the likely cable route, followed by a swop of the RCD to see what happens.

Anyway testing for a tripping RCD wouldn't need any form of equipment in most cases. What happens when the intermittent RCD is due to an internal DJ or some temperature related parameter?

If you care to ask anyone that does engage in faultfinding, I think you will find that the answer to most intermittents does not lie in the ability to test the items characteristics at any given time. The clue is in the words "nuisance tripping".

That is the reason for most testing. Have a look around a few new industrial installations sometime and see how far the rules are followed, yet every one has all the documentation signed off and catalogued before I'm let loose on it!

No! Apart from the odd PAT test I certify nothing. Anything I do certify is "by the book" and also safe to use.

If you know the current, and know the resistance.... It's called Ohms Law.

I didn't say I could do the checks with a DMM, read the post!

A wise decision. I like the thought of a logger, though.

I recently had a problem with an instrumentation feed. 110V and the Voltage was spot on every time I measured it. Clients supply, so it was outside my hands to investigate. All the 110V stuff and a lot of the 24V stuff, HMI, CPU, I/O cards and metering equipment had failed over the years. All I could do was replace the duff equipment which was most of what was on the PLC rack this time and fit a mains conditioner from RS. I did think about going for a UPS, but at one time it was difficult getting the client to appreciate there was a site problem.

Incidentally I remember commissioning the job a number of years back, and it was a constant fight with the electricians to cable the signal wiring properly, they had no concept of grounding and in desperation once, I went to B&Q to get some TV coax downlead just to demonstrate that my bits worked and their wiring wasn,t fit for puprpose. They never did wire it properly, when I last looked at the field wiring they had used twisted pairs to replace the original multicore SWA.

I know for a fact that these people must have had every bit of test documentation in place before departing or the project would not be in the hands of the current owners!

AB

[orion.osiris]

Excuse me butting in here, gentlemen, but i've been trying to follow this argument with some difficulty as a non electrician. What do you mean by "ELI"? To me that refers to the phase difference between voltage and current in an inductor which cannot be measured by a mere multimeter AFAIK. Clarification please!

[John Rumm]

Apologies, I thought it had been spelled out earlier in the thread, but it may not have been.

ELI = Earth Loop Impedance (aka Earth Fault Loop Impedance, and Zs)

When you have a fault to earth (i.e. a short circuit between line and earth), the current that will flow (the Fault Current or Earth Fault Current) will be limited only by the ELI.

This will be the sum of the impedance of the supply itself as presented at your house, the external impedance of the earth connection provided (Ze), plus the resistance of all the wiring between your consumer unit and the location of the fault.

The ELI is a fundamental metric that you need to know when carrying out circuit modifications / installations. You can estimate it by calculation, but this may constrain your design somewhat since you may have to assume a much larger Ze than is actually present. So its better to actually measure it with a loop impedance meter.

If the ELI rises too high, then you may not get sufficient fault current to open the protective device quickly enough[1] to give adequate shock protection, or adequate fire protection. If the ELI is very low at the fault, then you may need to consider if the fault current actually exceeds the breaking capacity of the circuit protective device itself.

[1] For example, a B32 MCB will require a fault current of 160A to operate in the "instant" part of its trip curve.

[Archibald]

And taking things to their logical conclusion, all those companies selling extention reels without the appropriate test equipment and safety notices should be prosecuted for negligence :-)

AB

[ARW]

No:-) Because the extention lead (in most cases) is protected by a BS1362

13A fuse and not a 32A MCB.

[orion.osiris]

On Sunday, 11 August 2013 17:49:17 UTC+2, John Rumm wrote: [...]

Thanks, John. In the electronics universe which I spend a good deal of time inhabiting, there is a mneumonic "ELI the ICE man" which prompted my query.

Take a butcher's:

[John Rumm]

Not if the lead has a BS1361 fused plug on the end... but it is one of the reasons you should not go daisy chaining multiple leads.

[Archibald]

Snip

Deep joy & happiness, all is well with the world :-)

In my opinion cable drums present a far more tangible risk of danger, yet they are freely on sale with only scant regard to warning people of the hazards.

A fuse has a response delay also. Methinks there is an argument for ensuring that the protective device on these things undergos a functional check at the point of sale. The storage system should be changed also. Optimum safety will only ensue when cables are sold in rectangular boxes of length equalling that of the required extention, with ventilation slots of course :-)

Seriously I have seen an"Asda special" after it was used to power a

2KW heater. It wasn't a nice sight.

AB

[The Other Mike]

Care to name the model number?

There are plenty of croc clips available that meet the requirements of IEC 61010 at various levels

For instance these are CAT III 1000V and CAT IV 600V rated

[whisky-dave]

"Reads voltage, electricity and resistance"

Oh look it reads electricity ????? always wanted to read that. :)