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Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Wed May 09, 2012 3:43 pm
by jonathan
I have just discovered that a large number of our monitors cannot be tested with the PrimeTest 250 due to them using external power supplies that have no metal parts on which to connect the earth bond clip. As the 250 halts the test sequence if the earth bond test fails, I cannot test these monitors or other devices that use a similar set up (such as laptops).

What are my options for this situation? The portability of the PrimeTest 250 helps a lot in my situation, I would like to have gone for the PrimeTest 350 but this is a massive leap in price for me and has many features I probably would never use.

Just as an addendum to this, in the training I received it is necessary to omit the earth bond test in some tests, such as those where there is no metal to connect the clip to, and these are considered perfectly legitimate and complete tests. As a basic tester, I would have expected the 250 to have this basic facility. Is it possible that the 250 could be updated in firmware (by an authorised Seaward engineer) to allow the user to press one of the buttons to skip the earth bond test?

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Wed May 09, 2012 8:26 pm
by Grizzly
If the monitors (and laptops) in question are supplied by an external PSU, then they are not Class I (or indeed Class II) appliances. They are ELV powered, and so it is not appropriate to be testing them using your PT 250, or any other Portable Appliance Tester.

The PSUs, however, will be Class I or II (although it may not be immediately clear which), and it's these that may attract a 'PAT' test.
Just as an addendum to this, in the training I received it is necessary to omit the earth bond test in some tests, such as those where there is no metal to connect the clip to, and these are considered perfectly legitimate and complete tests.

This may well be the case with some Class I PSU's that have no exposed conductors. They are not Class II, and won't carry the Class II construction mark, but they may be fully insulated, as the earth is provided for functional, rather than protective, reasons.

The wonderful world of power supplies!

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Wed May 09, 2012 8:41 pm
by Alan Freeman
On a couple of Class 1 laptop PSU's I have tested, I clipped my earth bond lead to the outer of the DC connector and had a good earth.

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Thu May 10, 2012 8:12 am
by jonathan
I understand that I am testing the power supply block, not the device it is attached to; that was made clear in my training.

As the PSU block does not have the Class II symbol on it and it has an earth pin, am I correct to assume it is Class I?

On a couple of Class 1 laptop PSU's I have tested, I clipped my earth bond lead to the outer of the DC connector and had a good earth.


I had wondered if connecting to the output stage would give a reliable earth bond test result, but didn't want to risk damaging the PSU by doing this; I had assumed that this would not be directly connected to the earth pin, and it sounds strange to me that it would give any kind of continuity to Earth at all. I had intended to contact the manufacturer for advice on how to test it, however I suspect they may not be of much help as it is an old model of monitor that does not even appear in their website support section. They may just tell me to omit the earth bond test!
I suppose manufacturers are not obliged to help with PAT test information. What if they come back and say that it should not be tested?

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Thu May 10, 2012 9:36 am
by jonathan
I have just read on another forum regarding testing laptop PSU blocks that they should be regarded as Separated extra low voltage (SELV) and only require a visual inspection. In the same post they also warn that performing a 250 or 500v test could pop the rectifier diodes inside the PSU.

If the PSU block is operating normally, supplying power to the laptop / monitor or what ever and not causing the casing to get so hot that it melts or smells of burning, and it passes the visual inspection, can this be considered a PAT pass?

As I understand it a PAT test is only showing the safe condition of an item now, it is not intended for predicting that an item might become unsafe in the future; along these lines, I have been testing some of our computers and got very consistent readings, however yesterday one of these computers gave a much lower than normal insulation test reading than the others, about 1MOhm; although it was still a PAT pass I knew it was not right and I could actually hear the power supply inside the computer fizzing. According to PAT, this item was safe to use, but I know from the readings and experience that the power supply will very shortly go pop of it's own accord (these units are almost ten years old, after all) so I removed it from service. I can tell when a PSU looks faulty or not from experience, can this be used as a basis for passing or failing an external PSU block in cases where electrical testing is not possible?

It seems that if the PSU could be damaged by running the tests and an earth continuity test cannot be performed because there are no metal parts exposed, nothing more than a visual inspection could be done.

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Thu May 10, 2012 8:01 pm
by Grizzly
jonathan wrote:I have just read on another forum regarding testing laptop PSU blocks that they should be regarded as Separated extra low voltage (SELV) and only require a visual inspection.


The PSU blocks may be supplying SELV/ELV, but they are fed from 230V mains electricity, so they are therefore either Class I or Class II.
They may only require a visual inspection (see HSE and IET guidance on that), but that is not down to them being SELV/ELV applianced (cos they're not). It is the equipment they are powering (laptop, monitor, speakers, whatever) that are the extra low voltage appliances.

In the same post they also warn that performing a 250 or 500v test could pop the rectifier diodes inside the PSU.


A 250V IR test will do nothing of the sort, and it's extremely doubtful if a 500V IR test would either.

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Sun May 13, 2012 11:18 pm
by Jim Wallace
Jonathan

If the power supply is not marked as double insulated it should be treated as a Class I appliance. Some PSU have continuity between the protective earth pin in the mains plug and one pole of the DC jack.

As Grizzly says, and insulation test should not damage any components as the test voltage is applied to both live conductors i.e. you are not putting 250/500V across the 230V input.

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Mon May 14, 2012 2:05 pm
by Grizzly
Jim Wallace wrote:Some PSU have continuity between the protective earth pin in the mains plug and one pole of the DC jack.


I've noted this mostly on Dell laptop power supplies.

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Mon May 14, 2012 7:50 pm
by jonathan
Thank you all for clearing this up for me, I just wanted some confirmation or clarification really.

Re: Omit earth bond test with PrimeTest 250 - possible?

PostPosted: Mon Aug 06, 2012 4:25 pm
by Jim Wallace
Grizzly - I've found the same. A number of people have contacted me regarding Dell laptop PSUs and reported resistance measurements anywhere in the range <0.1 ohm up to 100 ohm.