VC60B+ Megger for wire insulation testing decent?

[drakejest]

A friends home just got fully rewired and was suspicious that his electrician botched the wiring. He asked me if there was a way to check if the wires that are already in the pipes were somehow damage without pulling them out. So i suggested  to maybe use a Insulation tester or a Megger with the idea that if the electrician botched it so bad that the copper is exposed, the megger should read a lower mega ohm value.

With that said i cannot find to justify buying an so expensive megger that wont always be used ( maybe less than my fingers can count in 5 years).

The VC60B+ is priced relatively cheap that is tolerable for a device that might only be use once. Buy is it any good? or just utter garbage?

another that its about the same is uni-t ut501.

What do you think guys?

[Gyro]

A cheap insulation tester will give you some idea, but they are not the same quality as the ones designed for electricians (as you might expect)

A professional tester is designed to maintain its output voltage under resistive load, as defined by your local wiring regs. For instance, in the UK, the tester must be able to put out a minimum of 1mA without the voltage dropping below the rated test voltage, eg. If you measure a 1M leakage resistance on the 1000V range, it will be putting 1000V across that resistance, 500k on the 500V range, 250k on the 250V range etc. This guarantees that the leakage reading is actually the leakage at the voltage specified in the wiring regs.

The cheap ones may or may not sag, depending on quality (I had a Vici one previously that didn't even meet specified voltage on open circuit!). The indication should still be useful though, as you would normally expect very high resistance readings, hopefully much greater than 100M at 500V (although our minimum specification is 1M at 500V which is far too low!). The humidity in the Philippines might cause higher leakage readings, but for a full rewire, with fresh cable and fittings, the leakage should be very low.

Beware of accidentally spiking permanently wired fittings on the wiring - bathroom fans, LED lights, aircons(?) etc. These need to be isolated before insulation testing.

I've picked up cheap professional test equipment from ebay occasionally, when electricians are upgrading (going from individual testers to MFCs (multifunction testers) or utility companies are renewing their gear), eg. I have a Kewtech KT35 Insulation and loop continuity tester (has he tightened the connections securely?) which I picked up for far less than a new VC60 would have cost. Local conditions vary of course but it's maybe worth a look.

Note that such a tester can also be used for appliance PAT testing, very useful when purchasing things like USB chargers from dubious sources.


EDIT: Looking quickly at the specifications of the UT501 in various listings, it does look to be rated for 1mA output at the rated test voltages. The VC60B+ specs seem rather less detailed.

[Gregg]

Even a quality meg-ohm meter won’t tell you if there is a skinned wire not in contact with either the metallic conduit or an adjacent skinned wire unless there is moisture or some other way for leakage to occur.  An insulation breakdown tester might show a problem, but they are really expensive.  If there is a real short circuit, an ohm meter scale on a reasonable quality DMM will tell you as much as a meg-ohm meter.  Meg-ohm meters are more useful with things like motors, coils and transformers than for wire in conduit.
A better test may be a clamp on leakage tester like a Hioki 3283 with a built in low pass filter. 
You might look at this discussion:
https://www.eevblog.com/forum/testgear/clamps-meters-with-miliampere-(ma)-resolution/

[drakejest]

A better test may be a clamp on leakage tester like a Hioki 3283 with a built in low pass filter. 
You might look at this discussion:
https://www.eevblog.com/forum/testgear/clamps-meters-with-miliampere-(ma)-resolution/

That is an interesting suggestion, i did have a project involvng SCT-013 current transformer, and an ADC 16bit. Ill have to adjust the burden resistor to measure in the 1mA range. The ADC can measure at 50K samples per second. Do you think that setup will work so i do not need to buy a new tester.

What would i be expecting on the damaged wires? it will have some mA consumption on an open circuit(load removed)

[Gyro]

I don't see that you're going to be able to see such a low current with a current transformer. Even 100M insulation resistance at 240V would be a mere 2.4uA. Even with a 1:1 ratio current transformer, that's asking a lot.

I think the current contribution of the capacitance between cables with insulation in perfect condition and the conduit would swamp this out on AC mains. A capacitor with the same reactance as a 100M resistor at 50Hz is a only 32pF. Capacitance between wiring and conduit over a significant length is likely to be in the nF region.

I still think insulation testing at high voltage DC (to avoid the effect of capacitance) is the only practical method, it is, after all, what is used to assess insulation quality in installations. That unfortunately brings you back to one of the testers that you mentioned, preferably at the 1000V setting (although 500V is the normal spec for mains wiring) if you suspect nicks in the insulation.

[threephase]

I have tested 3 versions of the VC60B+. The first from KKMoon was very poor, failed voltage and current output tests as required by the IEC 61557-1 standard. The Vici unit, did pass at 250V, but failed on the 500 and 1000V tests, it did have the required short circuit current. Finally I tested the WinsPEAK version and it passed all the output voltage and current requirements on all three ranges.

All three versions had reasonable accuracy in regards to resistance values.

In terms of build, I did find the WinsPEAK unit to have the higher quality, probably the Vici unit next, the rotary switch on the KKMoon unit was a bit intermittent, but did improve the more the unit was used.

I do not have a Uni-T 501, so cannot comment on that one. I have tested other Uni-T insulation testers, 502, 505 and 533 and all were in specification.

The insulation test is usually the chosen method for verifying wiring before energising, perfectly acceptable to use. If you are concerned then you can apply a higher voltage as suggested by Gyro. As the wiring is dead when conducting the test, you can also move it about to see if the meter drops to a low reading.

Testing with an earth leakage clamp requires the circuit to be energised. The clamp is placed around the earth to measure the leakage current, so would need to be placed around either the earth core within the cable or around the pipe. If you cannot get to those then you move back up the circuit and clip round a bonding lead or earth termination in a board, the problem being then that you are reading the leakage from a larger circuit and don't know exactly where it is coming from.

The clamps usually have a 400 Hz filter, so you would need to add that if you were building your own.

Generally speaking we use the earth leakage test as a fault finding test when RCDs are nuisance tripping, it isn't recommended for verification of an installation.

Maybe you could try and borrow an insulation tester from someone or look at eBay for secondhand higher quality unit?

[drakejest]

A actual hands on review on an actual product is very very appreciated.  I do not know of any friends that have one and also ebay would be too expensive to ship to my region.

Good news is WinsPEAK brand is available in my region, uni-T is from what i know a somewhat reputable brand but from the reviews i saw they seems to dont like that the test button is a toggle type not a momentary type

I do not have a Uni-T 501, so cannot comment on that one. I have tested other Uni-T insulation testers, 502, 505 and 533 and all were in specification.

502 and 533 is also available in my area although x2 and x3 the price respectively compared to the 501/vc60B+

the 502 is on the grey zone on the budget. How would the VC60B+ compare to the 520 in your opinion?

Testing with an earth leakage clamp requires the circuit to be energised. The clamp is placed around the earth to measure the leakage current, so would need to be placed around either the earth core within the cable or around the pipe. If you cannot get to those then you move back up the circuit and clip round a bonding lead or earth termination in a board, the problem being then that you are reading the leakage from a larger circuit and don't know exactly where it is coming from.

We are planning to do tthis to check that his RCD is not near tripping,

I have not clamped an earth with a leak yet, what waveform am i expecting? assuming all appliance are turned off, is a perfect sinusoidal ?

[Gregg]

If you have a partially bare conductor inside the conduit that is not in direct or very close contact with the conduit (or a bare section of an adjacent conductor), even a 1000V DC meg-ohm meter won’t tell you for certain that a problem exists unless ionization occurs at the gap.  If the wire was pulled into the conduit using a water based wire pulling lube, you would have a good chance of finding a fault with a meg-ohm meter if you measured before it dried.  It probably isn’t advisable to run water through your conduit to achieve an ionizing environment.  Maybe there is some type of gas that would ionize much more easily than air and compare megohm readings with air vs. that gas would give some clue as to the insulation integrity.

Probably the easiest and cheapest solution is to use the existing wire to carefully pull in brand new wire and make up the connections in the same manner.

[threephase]

As far as I am aware, the UT502 utilises the same build as the 501, it just has a different voltage output range up to 2500V instead of the 1000V of the 501. It has the same style latching button for the on/off. It never bothers me personally as all my insulation testing is time based, at least 10 seconds, usually a lot more. You do get DAR and PI ratio tests, but these are for motor windings and not applicable to cable testing.

In comparison to the VC60B+, probably the same build quality as the WinsPEAK. It does have a case with it and a hard front cover that clips to the front of the meter. It did have higher short circuit current than the VC60B+. Probably not as accurate in terms of insulation resistance readings, but it will read up to 5.5 GOhm, whereas the VC60B+ reads up to 2 GOhm.

If you do have earth leakage, you will measure it as a standard 50/60Hz signal. In an ideal circuit you should not be able to measure anything as there will not be any leakage. If there is nothing connected but the wiring and you are getting mA leakage on a single circuit, then you would need to investigate more. As you add in more circuits to the test, the leakage can start to increase.

It will be dependent upon your actual physical setup. If this is a straight pipe, then the only cable damage you tend to see is caused where the cable is entering the pipe. You may see shavings of PVC insulation around the pipe entry that would give you an idea the cable has been damaged. The pipe entry should be smoothed and may have a bushing installed to protect the cable as it is pulled in. If this is not the case, then again, it would be more indicative of damage to the cable.

If it is a straight run and easy to pull the cable through, then you could just attach a draw wire/string at one end of the cable and pull it all out for a physical inspection. If it is ok, just pull it back in, if not arrange for replacement.

If it is a long run with bends in the pipe, then it is easier to damage the cable. The damage could also be caused pulling the cable around the bends, if the bends are pipe bends, then less likely, if they are actual pre-manufactured bends, then damage can occur if the pipe ends are not cleaned.

[Gyro]

Unfortunately a satisfactory high voltage insulation test (and actually not at that high a voltage, or very stringent resistance reading), in the absence of visible insulation damage, is the definition of 'acceptable' wiring condition in an installation. At least at the time of test.

Confirmation of the condition of inaccessible wiring is classed as a limitation of the inspection, as long as it meets the insulation resistance test.

It is difficult to have any sort of case against the electrician if the installation meets these inspection criteria. Hopefully, if this is a rewire, he has fitted RCDs or RCBOs which will quickly trip in the case of any exposed copper to conduit contact. Wiring in metal conduits is generally the safest arrangement for avoiding fire risk anyway.

I'm not excusing what may be a bodging electrician, but it will be difficult to establish a problem without effectively rewiring it yourself, ie. pulling new wires.

[jonpaul]

Hello: What country is this in?

Was a liscenced electrician used?

No cheap Chinese megger (or instrument) can be trusted if safety or compliance is involved.

As poor electric wiring can cause fire, shock or death, liability is apparant.

Merely testing wires with a hypot will not reveal poor connections, insulation cuts, wrong wiring, etc.

You should see the assistance of a licensed electrician.

Just the ramblings of an old retired EE


Jon

[Gyro]

The country is the Philippines.

Yes, agreed. The limitations of the cheap Chinese testers have been pointed out.

Presumably the electrician was registered in some way, the OP would need to ask his friend (whose installation it is).

Yes, the best (and most expensive) thing to do would be to have another licenced electrician prepare an Electrical Installation Condition Report (EICR in the UK). [Edit: Unfortunately, an EICR would still normally call out inspection of inaccessible wiring as a Limitation.]

Your retired ramblings are more concise than mine. 

[drakejest]

okay an update to the situation. there has been a change of plans and on one section of the house was redesigned, the electrical layout on that area does not match anymore and he decided to just pull everything out. Oh boy he was not happy on what he saw, the wires were lacerated all over the place to the point the bare copper can be seen on different section on the wire. I have no idea the electrician managed to do that. My friend and me seems to agree not risk it and have everything to be pulled out

We are DEFINITLY buying a megs tester, theres no way anyone can check several hundred meters of wires. The question now is which one we will get.

The plan is to try to salvage as many of the wire as possible. My idea on how to do that is to submerged the length of wire expemt for the two ends on water and measure the resistance with the meg tester. that way we will be able to tell if the wire have lacerations and throw that one out. those that passed will be used again.

[threephase]

Wire is usually damaged when pulling it through the pipe, known as conduit in our country, using a poor technique. Unless you have a short run, you usually need one at the start feeding it in and a second at the other end pulling the wire through. That way there is less strain on the wire and it can be guided into the pipe without snagging. Pulling a new wire into a pipe with wire already inside is another way damage can occur, or having too many wires inside the pipe, in my country we work to  40% spare capacity in the pipe.

I guess it depends upon the type of wire and quantity. If I had found damage, I would just replace the complete run with new cable, to give the most reliable solution.