Measuring Earth Leakage With DMM Without Clamp

I would like to know why I cant just use a DMM on current range with one probe on the laptop case and the other on the earth terminal at the mains socket.

This should (I think) work on a UK power system, but I am more concerned that you think you can feel a leakage current through the laptop case, especially since laptops have completely separate and double insulated switch mode power supplies and the laptop itself is running on extra low voltage (in that the voltage is not high enough for you to perceive any leakage current).

Are you sure it is not vibration or temperature you are sensing?

Thanks for your reply. My understanding is that switch mode power supplys (like in my laptops "brick") can use filtering capacitors which might leak a bit of current. Actually now I have been looking at it, I think the current is comming from my printer which connects via USB, but again that has a "brick" with probably a linear power circuit in it...but I suppose it could be SMPS. Can you say why a UK power system would be different from others regarding use of DMM to measure leakage current? Again thanks for your input. david

Many countries use different earthing systems. There are system(s) which isolate the supply from earth at the consumer point meaning you wont be able to get a return path back to earth (like when using an appliance powered off an isolating transformer).

http://en.wikipedia.org/wiki/Earthing_system

Actually now I have been looking at it, I think the current is coming from my printer which connects via USB, but again that has a "brick" with probably a linear power circuit in it...but I suppose it could be SMPS

It is possible (and modern printers use a SMPS), but the USB port should be electrically isolated enough to prevent this. The leakage current is generally due to the capacitor EMI filter that is connected between phase and earth and neutral and earth on most SMPS but this is generally a small and safe value.

Try disconnecting the printer and see if that fixes the problem, it is possible that it is faulty and may present an electrical shock hazard in which case it should be repaired or replaced.

jack of all trades. I understand now what you say about isolation at consumer point. So you think that in many cases I can just use a multimeter on current range to measure leak currents? I unplugged the low voltage power cable going into my printer and the leak is definitely coming from there, I can just feel it with fingers. I used my fluke 25 meter to measure voltage on outer part of low voltage feed and its 90 volts AC with respect to earth. Probably from EMI filter as you say, and probably needs fixing. Thanks for your help. david

I measured the leakage current too and my fluke says 139 microamps. david

The earth leakage current you may see quoted (or could measure with a PAT tester) on a "brick" type PSU would be between the live/neutral and the earth on the input (mains) side. There is usually a measurable leakage due to the capacitors on the filter (which is there to stop switching noise from the SMPS from getting back into the mains).

If the insulation resistance between any of the "mains side" connections and the low voltage side is less than 20MΩ (measured at 500Vdc), the PSU is almost certainly faulty.

Note that 230V across 20MΩ gives a current of ≈ 10μA. You can measure this sort of current (with a sensitive DMM) between earth and a wire coathanger (i.e. any kind of aerial) in a fairly electrically noisy environment.

John DG, Thankyou for your post which helps me understand this quite a bit. I used my megger on the printer adaptor and the insulation between mains side and low voltage side at 500v is over 100M ohms. So I take it this means that dispite the 138microamp between laptop case and earth...all is well. david

GA John. Well earned.

do you get the same result if you remove the power supply and run your "Brick" on batteries?

brich, "Brick" is a slang term to to refer to the block powersupplys that you gat with laptops and so on. No the tingling feeling comming from printers brick when plugged in. Thanks for you post.

Would this happen to be a Dell laptop? I had a housemate who bought one and you could clearly feel a current through the case - the replacement was exactly the same so must be a design flaw.

sulphurette, The currents from the printers brick but felt on laptop. I have a Panasonic toughbook. david

Try reversing the plug or wall-wart in the power socket of one of the interconnected pieces of equipment. Sometimes this helps due to reducing the phase difference in the power supplies and hence any capacitive action felt through lightly touching equipment. If it is felt when lightly touching and goes away when good contact is made the it is capacitive and not to worry. If, upon making better contact with the surface, the felling increases, then there is a direct ground leak which could be dangerous. You can't measure the voltage with your DMM probably because the small amount of current is not enough to be detected by your meter. Some meters have extremely high input resistance and amplifiers which can give a reading of miniscule amperages without "loading" the circuit down... virtually grounding out what was trying to be measured.