Down to Earth

[Drew1]

Can anyone tell me at what reading and on what scale would an item be considered down to earth? Therefore how many megs on 1000v or 500v scale?

[simbamolly]

The minimum meg is rated voltage plus a thousand, divided by a thousand.

[rocket]

attached PDF is Copeland's bulletin on megaohm

a meggar checks insulation not the windings

[gas_n_go]

If your checking for ground you need to find ground thats actually hooked up right then ohm out each winding between ground and that winding it you get continutity your grounded out.

[Drew1]

Thanks Rocket. Thats what I needed.
Most of the old meggars have a voltage scale that you select (500v or 1000v). What is this for , because Ive bought a multi-tester with digital auto ranging which measures from ohm to mega ohm automatically. Is this still classified as a meggar? and can I use it to test down to earth? Therefore if I use my multi tester on auto range and I get a reading of <0.5megaohm is that compressor condemned or do I need to buy a special meggar (500-1000v) to test it correctley?


Adding to that: At what megaohm reading would a compressor cause a standard household earth leakage unit to trip.?
And why are earth leakage units not rated in megs of resistance. Therefore we could say that a earth leakage which is rated at 0.5megs would trip when the compressor reaches this point?

[Drew1]

From what Ive been reading you need to increase the voltage in the system to find the "leakage". A standard multi tester doesnt increase the voltage enough??
Therefore we need a "megger"??

[clgman]

Not a wicked electrical guru - but I'll give it a go !!! A megohmeter actually induces a DC voltage into the device being tested. Usually it is done at twice the actual AC voltage that the device operates at, thus a 230 volt compressor would be tested at the 500 volt setting, 460 VAC tested at 1000 volts, etc, up the scale we go. Now, the readings can be all over the scale depending on what you are testing. Generally, most of the time, small motors, wiring, etc., will test out very high, 100 megohms or greater. The weaker the insulation value, the lower the reading. You must know what an acceptable value is for a particular motor or device, or you might condemn something that is still good, or vise-versa. Again, it depends on what you are testing, so it might benefit it you to develope a collection of readings on equipment that you typically work on, so you have a feel for the "norm". So when possible, check the manufactures specifications. Good luck !!

[Drew1]

Thanks for that! As with refrigeration this topic doesnt seem to be an exact science. What you mention seems to be what most people talk about: Apply 2 x voltage and see that your resistance is above 0.5Megs to earth ( copeland)
Others say you should be above 20 megs?

[clgman]

Thanks for that! As with refrigeration this topic doesnt seem to be an exact science. What you mention seems to be what most people talk about: Apply 2 x voltage and see that your resistance is above 0.5Megs to earth ( copeland)
Others say you should be above 20 megs?

That 20 megohmeter reading - is directly related to what you are testing - some new larger motors test out at 1 (thats one meg) from the factory. This would be a new compressor motor. So, again, the "norm"for a particular device must be known. I would agree that if a device typically tests at 100megs or higher, and now tests at 20 or below, and trips the breaker or requires a new breaker setting, then it needs to be condemned. Justmy opinion !!!

[Drew1]

At what mega ohm reading would a standard earth leakage system trip on? I now understand that different products can have different acceptable down to earth readings, but at what point would it be dangerous ?

[gas_n_go]

At what mega ohm reading would a standard earth leakage system trip on? I now understand that different products can have different acceptable down to earth readings, but at what point would it be dangerous ?

I would like to build on this question if i may. I own a megger but rarely use it. I normally just ohm it out and either its grounded or it isnt, and kinda run with things like that, but if I were to start incorperating using my megger, and found that the varnish was starting to wear to a point that would be considered below acceptable tolerance range. Would it be feasable to pull that motor and send it out to have the windings revarnished. Im just curious if I have a compressor in good shape with the exception of the windings if they can be pulled and revarnished and reinstalled and be able to get extended life out of my compressor that might make it worth keeping a meg ohm record on compressor as a matinence program.

[Drew1]

With a normal multimeter (even though it still measurers Megs) it doesnt have the pressure( volts) to 'leak' down to earth? Therefore we need to use a megger (1000v).
So a multimeter reading of 1megaohm and a megger reading of 1megaohm(1000v) are not the same????

[hvacpope]

Drew,

take a minute and read this http://www.termogram.com/articulos/U...g-Don_Shaw.doc and this http://www.pdma.com/InsulationFaultZone.pdf

your meter with megaohm features is as good for meggin as a dedicated megaohm meter, you dont want to go crazy meggin so often cause you may end up weakening the motors insulation.

[brdman]

I still am alittle unsure of copelands saying if you have 208/230v =readings of .208-.230, 460v= readings of .460 megohms,and then at the end what I'm confused then to have the article say <.5 condemn? any help with understanding this appreciated.

[rocket]

Copeland has set the limit at 0.5 megohms before a compressor is condemned.

I haven't seen bad winding insulation on a Copeland Compressor since 1970
so all this mute. Some use multiple readings to determine moisture problems
but as far as testing insulation breakdown, it's really no longer necessary.
Insulation has improved a great deal since the early 60's

Your multiumeter can test ohm winding restance against specs as far as checking the windings are concerned. Unless your checking moisture in a large chiller, you can retitre your megger

[gas_n_go]

Copeland has set the limit at 0.5 megohms before a compressor is condemned.

I haven't seen bad winding insulation on a Copeland Compressor since 1970
so all this mute. Some use multiple readings to determine moisture problems
but as far as testing insulation breakdown, it's really no longer necessary.
Insulation has improved a great deal since the early 60's

Your multiumeter can test ohm winding restance against specs as far as checking the windings are concerned. Unless your checking moisture in a large chiller, you can retitre your megger

the only thing I can add to this post is when my mom was pregnant with me in 1970, she asked the doctor to take a meg reading. He told her I was faulty, and Im sorry to report he was right.

[Drew1]

Thanks hvacpope. That link is a good read.

[clgman]

Copeland has set the limit at 0.5 megohms before a compressor is condemned.

I haven't seen bad winding insulation on a Copeland Compressor since 1970
so all this mute. Some use multiple readings to determine moisture problems
but as far as testing insulation breakdown, it's really no longer necessary.
Insulation has improved a great deal since the early 60's

Your multiumeter can test ohm winding restance against specs as far as checking the windings are concerned. Unless your checking moisture in a large chiller, you can retitre your megger

There are fault situations that definitely require the use of a megaohmeter to locate an active hard to read fault, or a near potential fault. Case in point, commercial / professional office park type building, equipped with twenty - 25 ton RTUs with data resource areas. Site has ten minutes of UPS available for data areas. Building main electrical service, one 2500 amp ground fault type breaker. A 1/2 HP condenser fan motor suffers a line to line short in a rain storm in one of the RTUs and trips the main breaker to the building. Now it also opens the fuses in the fused disconnect on the unit. The main breaker gets reset - the fault is already cleared from open service disconnect fuses being blown. Troubleshooting takes place the following day when it is sunny and dry, no apparent faults anywhere to be found , except by use of a megaohmeter, testing all the loads in that unit. Could not be found with only an ohmeter, even a qaulity one with high resistance scales(Fluke 87 III).True story. This is offered up only as a situation that requires testing with a megaohmeter, I'm sure there are many others as well. Just my opinion !!!!

[Drew1]

When a compressor bearings have worn a bit and during running we have the stator touching the rotor the system would go down to earth to a split second??
When we then meg out the motor it could show clear cause the rotor is not touching the stator? How can we diagnose this fault?

[hvacpope]

When a compressor bearings have worn a bit and during running we have the stator touching the rotor the system would go down to earth to a split second??
When we then meg out the motor it could show clear cause the rotor is not touching the stator? How can we diagnose this fault?

by turning the motor shaft with your hands making sure it moves free and there isnt a whole lot of axial movement, the motor will be very hot.