Stupid earthing question

clarkefan

When I measure earth from a point on the PCB in my amp to chassis I get a reading of 0.2ohms.

So, stupid question, is this to be expected?

My brain says it should be *zero*, or does even a little earthing connector lead mean there will always be some minor residual resistance?

ecc83

0.2 Ohms is actually quite low! Most DMMeters will show a minimum reading of around 0.5 R due to lead and other resistance. If you really want to measure very low resistances you need a special meter with 4 wires (sense current and read wires)

From a SAFETY point of view the chassis to plug earth pin should be no more than about 1 Ohm but the only proper way to test this is with a PAT unit as a high current needs to pass.

As a bit of (probably useless!) information, if checking transformer windings (ALL disconnected from power of course!) you get a silly reading or one that won't settle, short out some or all of the other windings.

Dave.

ICBM

If you touch the probes together do you still get the same 0.2 ohm?

clarkefan

ICBM said:

If you touch the probes together do you still get the same 0.2 ohm?

Er, just checked and er yes I do (sheepish grin)

Thanks, in the 15 years I've had this meter I've never seen that

And thanks for your thoughts too @ecc83, now I'm wondering if the reading should be higher!  My meter is a big Maplins multi jobbie, nothing expensive I assure you