Insulation testing

[lorano]

When you guys are doing a service/maintenance do you use a megger?On machines that have been poorly looked after for example have dirty coils,hot running compressors i always get very low readings(below 250 Mohms).incidently I tested equipment that hadn't been used for years so was cold and they to had very low readings?!The machines I'm on about are fridges and freezers.

[chemi-cool]

If it is running, [poorly or otherwise] nothing is tripping, I dont see any reason for a megger testing.

[lorano]

I've never used a megger in previous jobs but my new work place insist?Like I said a few machines that I've been to have been in poor condition and running 'hot' due to dirty condensers.Im convinced that this messes up resistance readings.Also units that have been dormant for months and just switched on give false readings as when they 'warm' up the resistance readings are above normal.I don't see the need for it?!

[Magoo]

I am astonished with previous comments, " never used a meggar, if it runs why test ". Multi-meters are a joke if used for ground leakage testing. I have had megger variants for all my years in the trade
Ever heard of criminal liability if a client is injured or at worst killed by an appliance that you have recently serviced.
I bet that no has ever done a loop impendence circuit test on a new installation.

grumpy magoo

[anggoro]

Dear All,

In my plant, usually we will test the insulation resistance of the cabling and motor after the equipment is idle for quite some time.

We had case (a few days ago in fact) where the equipment had been idle for quite a prolonged period and when it was tested, the reading is low. We tried to test-run the equipment and the motor got burnt.
Therefore, I think it is better to test it first before operate the equipment.

Anggoro YN

[chemi-cool]

I am astonished with previous comments, " never used a meggar, if it runs why test ". Multi-meters are a joke if used for ground leakage testing. I have had megger variants for all my years in the trade
Ever heard of criminal liability if a client is injured or at worst killed by an appliance that you have recently serviced.
I bet that no has ever done a loop impendence circuit test on a new installation.

grumpy magoo

Please dont get grumpy, have a cold pint and relex.

Megger testing is a way to fined faults in electrical circuits.
Loop testing is to make sure eart impedance is ok and RCD will protect in case of a fault.

I always check new electric instalation with a megger and the sparky who supply the currant to the new electric board must check loop testing.

i dont do regular megger testing but i do test RCD for proper function.

[Frikkie]

I would recommend you don't check compressors with a mega tester. The high voltages coupled with the fact a mega tester uses a modulated DC test output can blow any thermistors that might be integrated with the motor windings for thermal protection. The junction of most solid state devices won't survive a mega test unless it's specifically designed for it. The same would be true for any machine that has electronic controllers built in. If you really must do a mega test then I'd only use a 250volt test and definately not on the 500v, 1kV or 5kV range and always have it in mind that this kind of testing can cause faults as easily as it can find them.

With regards to seeing leakage, it's also worth thinking about what else could have an effect on the test results. Most modern day items have noise supressors and surge/spike arrestors built into the early stages of the power supplies. These devices all have a clamping voltage that could easily be exceeded during a Mega test and as soon as they start to conduct they allow a current to flow to earth. This will lead you to believe there's a fault when there isn't because the earth is a functional connection as well as a protective one. Also altitude can effect insulation readings so the pressure (or vacuum) in a refrigeration system will almost certainly have a hand in the results seen during testing.

An insulation measurement of 250Mohms mentioned in the first post is 250 times higher than the figure of 1Mohm which is generally considered as the absolute minimum requirement (assuming the result was obtained at the normal running voltage). Why do you describe the result as low and why do you think it needs addressing?

If the concern is for user safety I'd suggest maybe rather focus on making sure the circuit is going to perform if a fault occurs. An earth impedance test and an RCD test would be of value for this.

[Grizzly]

Frikke.
Your point may be relevant on small stuff?
But not on most of the kit i work on.
Even the motor manufactures sometimes give expected values.
If you are worried about frying components don't do it.

But I suspect most on this forum know what they are doing.

The trick is to disconnect that which you don't want included in the test. Meggars are a valuable tool in the correct hands.

So I have to disagree with the bulk of your statement.
Grizzly

[Frikkie]

Hello Grizzly.

I agree a mega tester is a valuable tool in the right hands but I fear there are many hands they should not be in. You are also right I was thinking of smaller dome compressors as well. Please forgive me if my advice is misleading and I certainly wouldn't imply the people of this forum don't know what they're doing. I usually type at the same time as I think and my English can be difficult so sometimes my posts can be somewhat chaos.


I am still hesitant about how much value they would be as a tool for general refrigeration servicing or maintenance. You could use an earth leakage clamp meter around the live and neutral wires simultaneously to get a precise value of current leaking to earth under actual running conditions, this would be more accurate and it wouldn't have any of the destructive risks the using a mega tester would have.

You can do this clamp test on a three phase machine or motor as well as long as you clamp all the wires including the neutral if one is connected.

[mark957]

After many years of testing with megger insulation of compressors, do tests and recorded periodically, we can monitor the increase of moisture in the circuit and the insulation of the compressor was not only to earth but also between windings part winding, so from my experience if the megger is used with knowledge of the facts is a useful tool for routine maintenance

regards Mark

[GHAZ]

Always use meggar on a cold compressor and check ohms resistenances because when compressor has been running the resistenance goes weak with heat. I did check once a compressor with 1000v and at same time put a mutimeter on the thermal overload pins which were connected to kriwan therms and i got induced voltage of 36volts dc.but nothing happen to them. And yoh are not suppose to put more than 7 volts. There fore when machines are running with 400v. Thr thermal overloads are getting induced voltages

[cateyes]

When I was first trained to use a megger 250 meg ohms was considered more then acceptable by manufacturers standards. Machines with these values ran fine for years afterwards and no change was noted during subsequent inspections. This isn't implying changes don't occur and the reason why they're done.

[nike123]

Yesterday, I was testing insulation resistance on one Maneurop scroll compressor SZ090S4VC which is more than 5 years old and failed mechanically It has seen lot of overheating since paint on dome is wrinkled and brown. Insulation resistance was measured with 500V and after 15 sec it is stabilised at OL (out of measuring range wich is 4GΩ at 500V test voltage for used instrument Uni-T511) for each winding.
Today modern winding insulation resistance could reach number of 20GΩ in new product and during working life of that product could stay easily in region above 1GΩ.
Below that number we have indication that insulation or condition in motor is deteriorating more than it should be.

Therefore, it is of great help to make insulation test of motors at comisioning and note measured data for future reference.

By new standards IEEE 43-2000, not aceptable winding insulation is below 100MΩ.

http://www.transcat.com/technical-reference/IEEE.aspx