Pat testing help

[goodconnection]

I'm taking C&G 2377 22 next year probably around May ish My hunch is that asking some qualified electricians a few somewhat daft questions may give me a better idea of what my strengths and weaknesses are with regard to my current understanding of the In-service Inspection and Testing of Electrical Equipment. I'm qualified in IT HNC level so maybe there is some hope eh !

I'd like to do a good job and have enough background knowledge to be in a position to fully understand the results obtained from all the tests carried out and if necessary advice duty holders if a qualified electrical engineer is required to investigate further. I hope you get the general gist of this ....


I currently have a Metrel gamma pat battery powered device and as such lack the ability to carry out certain tests.
my purpose in buying one has been to observe the functions of the device it's features how it presents it's results and the opportunity to practice carrying out basic tests in the proscribed manner on pieces of my own equipment.

Note to moderators: please move the posting to wherever you feel this belongs in the forums ....

Ok this is the end of my preamble and up next is question 1.



Laptop Power Supplies .... how to keep end users safe !

I do IT and have seen hundreds of the blessed things and to be honest many get bashed about and lots seem to malfunction.

In the UK we appear to have two versions

type a class1 device enabled with a protective earth conductor and a class 1 3 core cord set onto a BS fused 3 pin Fused Plug
type b class 2 device enabled with double insulation live/neutral input and a class 2 core cord onto a 3 pin BS Fused Plug

similar sort of arrangements for some peripherals including printers ....


does class 1 AC 2 DC power supply = Sensitive Device = 100 mA / 200mA Earth Bond /Continuity test parameter ?
but how to check it when there in NO obvious means to attach the probe . only metal surface point available is the negative line of the dc output terminal that supplies the laptop.

A good visual check will obviously pick up signs of damage to the moulded case of the Power supply and any chemical invasion or otherwise signs of trouble eg melted / smelly polymer/ overheating the cord set is treated as a class 3 device and tested as such.

I suppose the first thing to be done at the start of test would be to deduce if the equipment and the environment that it was being used in and the purpose of use was suitable from the view of safety ....

are there any circumstances where the PS can output high voltages through the dc line ? and if so what would be the cause ?


Please feel free to comment and throw in your views and experiences of dealing with such matters in your course of business I'm grateful to the forum for providing this facility and hope that actually making a connection with those with far greater knowledge and abilities will help me in my learning process.

 

[Marvo]

.........Note to moderators: please move the posting to wherever you feel this belongs in the forums ....

I've moved this thread to the PAT testing forum for you.

 

[Lucien Nunes]

If the PSU DC output is grounded (and some smaller ones are not), I often do a low current test just to prove the CPC continuity without necessarily testing its robustness. In many cases the earth is functional only and not needed for fault protection. It usually serves to collect leakage (e.g. from RF filtering) that would otherwise raise the DC output to sit on half mains voltage if the load is well insulated from earth. The source impedance of this voltage is too high to be dangerous but it can be annoying, you get tingles off laptops etc due to the (typically 0.1- 0.5 mA) of leakage.

In the event of an internal failure of the input/output isolation, the CPC connection might save the output swinging up to hard mains, although in normal usage that is ultra-rare. I've been handling small SMPSUs under really adverse conditions for years and doing post-mortems on them and never seen it happen once yet. But I still test ground-through where I can (because of the environment some of my equipment works in) and of course with a class II supply the insulation test between input and output is essential.

Out in the real world, you would be amazed how many would test a class II PSU by plugging it into the tester, leaving the wander probe on the table, and pressing 'Test'. Yet more would just put the sticker on without unpacking the tester, which is actually better as it doesn't show electrical incompetence. OTOH this morning I found pass stickers on a USB-powered device, a Scart cable and part of the fixed installation...

 

[goodconnection]

Thank you Lucien for your valuable insight. I started with laptop / appliance AC/DC PSU's for a reason and it's an interesting story and one of the factors why I am trying to improve my electrical knowledge and practical abilities regarding equipment failure and safety issues. I on occasions provide assistance/support to those who are visually impaired and or severely disabled. Anything in the way of PAT items for this client group could be a real danger if said item was damaged. One person recently had a Power supply failure where the casing split open exposing the wiring and live parts and I guess it was only luck that saved them from contact with the 230 v ....

I also have in my procession a laptop supply that has gone the same way it's casing has split again exposing the inner workings. All made in the same location and you do not need to have a knowledge of Ohms law to work out where.

It was interesting reading your comments on the real world and to be honest I have mixed feelings about my own observations regarding how some PAT operators conduct themselves out in the field. I have an image in my minds eye where such a person might try putting a PAT item into the freezer for a few hours in the hope of inducing super conductivity in order to achieve an acceptable earth continuity reading and thus be able to attach a pass sticker !

Moving on from question 1 I would now ask how would you test a laptop or similar piece of IT ? eg something that is DC supplied IET guidelines and other legal related stuff all point towards employers and users having to take responsibility for their own safety. I have encountered many Laptops with a pass sticker on them and wondered what would be the "Lucien Standard" for you to afford a device the same honour ? I guess those that passed the equipment on previous occasions had just visually inspected it ..

 

[goodconnection]

Many Thanks

 

[Lucien Nunes]

I haven't dealt with general PAT for a long time, so your best practice might differ. I only meet it nowadays in my specific field where equipment is used by instructed and skilled persons but subjected to heavy wear and tear.

If I found a laptop with a pass sticker, I'd peel it off. Class III devices are outside the scope of our testing and there could be confusion if a device with a pass sticker was used with a PSU that hadn't been tested or had failed. We consider PSUs as stand-alone items as they are often stocked in large numbers and the first available one picked out of the box and used with the first available device. In our industry they don't live together as pairs.

 

[goodconnection]

I'm glad of your answer ... My reasoning was that a laptop was essentially a battery powered device that could be charged up when and if needed and therefore not really subject to PAT.

Of course the 230V supply is the potentially dangerous item and is !

 

[goodconnection]

A question about Pat Testing involving Washing machines / Tumble dryers / Irons etc etc

Need to understand a principle about the logistics of the earth leakage test and how the result is presented by a PAT test instrument.

Observation: I brought myself a plugin type energy meter from Maplins a while back that essentially plugs in before a device and provides usage telemetry in real time regarding Amps, Watts , Volts and VA readings.

I now have a fair idea of how many AMPS my washing machine and other household items need to draw from the fixed installation for the various programmes / setings depending on what is being asked of the equipment.

OK my question is thus if I carry out a class 1 earth leakage at Mains voltage (powered up) on a Washer and energise all the device circuitry how does a PAT test device work out if it exceeds the pass level. Say I just turn the washing machine on and it's not heating water it's on at a basic quick wash thats drawing less Amps would that give a false result ? do you have to test the machine at the point where it draws the most current and therefore at the time when it could be dangerous to the end user ?

Insulation resistance and it's ability to protect against leakage to my knowledge can be dependent on various factors Moisture , Humidity, Temperature, Atmospheric pressure and or a combination of all ........

I don't have a PAT Tester capable of a 230 v Earth Leakage test at this moment My Metrell has only a Sub leakage option due to the fact that it is battery powered.

How do the more complex test instruments work out the correct Earth leakage current reading is it just whatever it finds when the machine/appliance is switched on eg cold wash /1 bar on a 2 bar electric fire / low heat setting on a tumble dryer. or am I missing something ?

Thanks for any help you may be able to offer !

GC

 

[telectrix]

this is one area where PAT falls flat on it's face. you can only plug the machine into your tester and do the tests as it is. in other words, machine may well give an IR reading that is perfect, yet a faulty heater may trip RCD when machine goes into heat cycle.

 

[goodconnection]

Thanks telectrix I thought as much ..

No advice in the IET COP to use brain in any particular way just a list of fail pass limits ...

So turn dial press button on washing machine press test on PAT instrument and get protective conductor current leakage result that is probably not the correct reading unless you know which part of the washing programme and or cycle hits the max Watts rating and Amps on the plating.

Hmmm Yes earth leakage testing at nominal 230 v in respect of washing machines would appear then to be inherently inaccurate.

Washine machine manufactures need to put an electrical max watts test cycle shortcut on their equipment or rather BS standards should demand one eh !