test for short ??

[onlyfitidealboilers]

need some help pls..

after my last post "positive isolation" got me thinking

say if am cheaking for a short between the main live an neutral to the programmer boiler etc but what about thigs with relays like three port vlvs & programmers

i would have to check the switch lives also i guess

Help my head is up my arse now!!

 

[londonboy]

You should really disconnect everything, and use an insulation resistance meter, which tests at 500v.

This meter will damage electronic components etc, hence the reason for testing in isolation.

 

[automationman]

If you are checking for a short circuit then you first need to define from where to where and are you really looking for a short circuit or poor insulation resistance.

Examples: from Live to Neutral over a 6 meter length of open cable.
from Live to earth over a 4 meter length of open cable.
from Live to Neutral of a 7 meter length of cable connected to
a 100 watt incandescent lamp
From Live to Earth over a 3 meter length of cable connected
to sensitive electronics equipment.


In the case of the open cables then you can use a MultiMeter on the low ohms scale or the low Ohms range on a multifunctional tester. You would expect a very high reading near infinity or at least above 1Mohm (though 1 MOhm would still require further investigation in an open cable)
If you did get a reading of infinity then you might at that time use an insulation tester to check if there is insulation breakdown/damp etc in the cable. The point here is that you need to think about what you are looking for, short circuit or insulation resistance? They should be considered as different parameters when fault finding.



In the case of the circuit with the 100 watt lamp then you would expect to read the lamp DC resistance which may be in the range of 300 to 500 ohms. If you obtained a reading of say 2 ohms then you might suspect the lamp or the cable. you might then remove the lamp and expect the reading to go to infinity or at least above 1Mohm.

In the case of the sensitive electronics, a multimeter will not damage the electronics (an insulation tester would probably damage the electronics) but it is difficult to predict what DC resistance to expect unless you have exact details of the electronics circuit so best disconnect and take your resistance reading which you would expect to be infinity or at least above 1Mohm.

An insulation tester is used to test for insulation breakdown either in the cable, at cable terminations or inside electromechanical devices that do not contain sensitive electronics. It is not the best instrument to use when looking for short circuits which are typically quite low resistance faults. It will not have the desired resolution at the low ohms range.

 

[holmslaw]

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[automationman]

I'm not going to dig through all of this nonsense, but suffice to say a dead short on a circuit has exactly the same effect and is the same thing as an insulation failure.

It does not have the same effect and it is not the same thing.

Example: (assuming 230V) insulation resistance between L/N of 5KOHM would result in a fault current of approximately 46 mA which would not activate a 20amp MCB or even blow a 1amp fuse. A dead short would result in a very low resistance and almost certainly activate the protective devices. This simple symptomatic difference and more are considered by experienced electricians/electronics engineers when fault finding.

I will ignore your rudeness (one more time) but will report you to the moderator the next time you try this with me. The reality is that I probably have many more years of electrical fault finding experience and qualifications than you do and almost certainly more diplomacy. You can contact me privately if you wish to debate this topic and your experience. If I remember correctly, the last time you did this to me I invited you to come and work with me for a day or two. You are still welcome.

 

[holmslaw]

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[automationman]

If you have electrical experience and knowledge then share it on this forum with dignity and debate issues with respect not with rudeness.
Who is it you are trying to impress?

 

[holmslaw]

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[ericmark]

need some help pls..

after my last post "positive isolation" got me thinking

say if am cheaking for a short between the main live an neutral to the programmer boiler etc but what about thigs with relays like three port vlvs & programmers

i would have to check the switch lives also i guess

Help my head is up my a**e now!!

This is a point I bring up again and again when people talk about PAT testing. Central heating, Washers, Even freezers have timers and thermostats which only connect some items for a short time.

Where twin pole relays are used the only method of testing involves some dismantling. But most items only switch the line so testing neutral to earth will in many cases highlight faults even when the de-frost element, motorised valve, empting pump etc are not energised.

The other problem is earth wires on printed circuits. Many PAT testers draw 25 A to test the earth and this can burn out the earth track on a printed circuit.

Already mentioned the 500 v used for insulation testing can also cause problems. However since the item should be disconnected from supply before testing so there is no link between neutral and earth on many items 500 v earth to neutral or earth to live will not cause a problem. Except where there are EMC filters on supply.

So with electronic items one has to be careful and using 200ma for earth continuity and 230 v for insulation resistance is a wise move.

But to test any equipment one has to know what it has inside and if it is regarded as IT equipment or not. And the PAT testing course I did was far too short to teach non electrical people how to safely test "In service electrical equipment" without damaging the odd appliance from time to time.

Even an electric drill has a speed control which is electronic. Vacuum cleaners have EMC filters and other than the standard lamp one questions what one can use 500 volt with out risk of damage.

For an electrician to do a test and write out the procedure and then for a semi-skilled to follow the laid down tests is OK. But to expect non electricians to test must mean you will also expect to replace things

 

[Stoday]

In the case of the circuit with the 100 watt lamp then you would expect to read the lamp DC resistance which may be in the range of 300 to 500 ohms. If you obtained a reading of say 2 ohms then you might suspect the lamp or the cable.

What if you got a reading between 30 and 50 ohms?

The fact that the cold resistance of a 100W filiament lamp is around 40Ω and not 400Ω suggests that automationman's practical experience is not very extensive. Hence his suggestions should be taken with a pinch of salt.

 

[automationman]

Arggghh, you have both caught me out, 34 years in industrial control
is just a drop in the ocean compared to your experience. Just off to test the gate/drain of some power MOSFETs with my 1000volt insulation tester (have jammed the test button on with chewing gum for quick fire action)

Now that you have brought me to justice (well deserved too) why not take some time off and provide helpful technical advice on the forum?

 

[holmslaw]

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[ericmark]

I think the problem is to learn how to test and inspect one needs years of experience in electrical work then a course or two on testing.

For the first 10 years I did not need to inspect and test. Then I got a job where I had to do the testing. I remember getting the earth loop impedance meter looking at the reading and realising I did not have a clue if good or bad.

It was a case of pulling out all the old books and reading my old college notes and then decided better do my C&G 2391.

OK some people are swats and learn everything taught at college but I remembered it just long enough to pass exam. Didn't need it with job I was doing then.

Pobodies Nerfic and we all make errors. At least "onlyfitidealboilers" is trying to learn. I think he would be better taking a course but full marks for trying.

Now with 40 years experience I realise there is loads I don't know. After finishing apprenticeship I though I knew it all.

And these people who think they know it all are especially annoying to those of use that do?

 

[automationman]

ericmark, thank you for bring common sense and decency back to this thread. I completely agree with you that for many people (possibly most) it takes a few years of hands on practice then some formal training. In fact, I think that should be the way because I have seen that method produce very good technicians who have good practical experience and a new-found enjoyment of getting to grips with the theory. in my 1 year plus 33 (to make holmslaw happy) I have learned something new almost every day. Sometimes fault finding can take a person on a departure from pure logic and it become a matter of instinct and intuition. Though some such departures have taken me down a blind ally leaving me with philosophical clarity that I should have stayed on logic lane

 

[holmslaw]

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