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Earth continuity tests PCs and VDUs

PostPosted: Tue Feb 15, 2011 2:11 pm
by edutech
We have come across a number of LG monitors and Dell PCs that are failing the earth continuity test in so far as the readings are above the threshold. We are reluctant to fail them as this would cause considerable disruption to the client. We have tested the appliances at several different points with the same result.

The question is: do we fail them or is there some feature with this appliance that would give the high readings?

Re: Earth continuity tests PCs and VDUs

PostPosted: Tue Feb 15, 2011 10:38 pm
by Grizzly
At what sort of reading do they fail at?

Don't forget, the earth continuity pass limit is (0.1 + R) Ω, where R is the resistance of the protective conductor of the supply cord/lead.

So if for example, they were coming in at, say, 0.12Ω, this may still be a pass, as the mains lead might be contributing the 0.02Ω (You would easily know this, as you'll have already performed the seperate test on the detachable mains lead, right?).
However, if you were getting readings of, say, 0.5Ω or even greater, then it's doubtful that the mains lead is contributing that much (again, easily confirmed), so it probably is a fail.

All described in the IEE Code of Practice, section 15.4, pages 73 & 74

Re: Earth continuity tests PCs and VDUs

PostPosted: Wed Feb 16, 2011 8:05 pm
by Jim Wallace
Hi Edutech

If exposed metal parts are connected to protective earth for the purpose of protection against electric shock then there should be a low resistance ( as per the limits in the IEE Code of Practice) path between the exposed metal and protective earth in the mains plug. I have come across IT equipment which has 10's of ohms in the path between exposed metal parts and the protective earth in the mains plug, with no other means of protection (e.g. double insulation). In this case, if the basic insulation failed there would be a real danger of electric shock. Manufactures are not permitted to place components in the protective earth path which increase the resistance above permissible limits.

The metal parts maybe connected to earth for shielding / EMC purposes (rather than for protection against electric shock) but the only person who can give a definitive answer is the manufacturer of the equipment.

One other problem that I have come across in the past is that manufacturers wind the protective earth (inside the appliance) around a ferrite core to comply with EMC regulations. This forms an inductor (coil) which presents an impedance to AC currents. In this case, an AC test current can give a reading higher than the limits in the IEE CoP. This can be confirmed by performing a DC earth test which will give a lower measurement.

Re: Earth continuity tests PCs and VDUs

PostPosted: Thu Feb 17, 2011 11:39 am
by edutech
Thank you for the response. The earth continuity tests for the appliances were quite a bit above the threshold and the technician used a short IEC to connect with a pre-calibrated resitance value, so I guess it must be a fail.