Explanation of Insulation Resistance

[chalkmycue]

Hello

I am hoping somebody could give me a bit of information regarding insulation resistance testing.
My query is really regarding insulation testing when PAT testing i am experimenting using a Low resistance ohmeter with a Kewtech PAT adaptor

My understanding is that you apply a voltage of 500v to an appliance(reduced to 250v if 500v would damage the appliance) you then get a reading on the tester the higher the better my tester is giving me a greater than 999M Ohms on appliances i have tested within my home.

My main question is what would cause the readings to be low is it just the insulation deteriating or can there be other faults? if so what are they and secondly could i simulate a fault using a length of cable with a lamp on the end to give me a better understanding.

Thanks for your time.

Ricky

 

[Johnmelad502]

Hello

I am hoping somebody could give me a bit of information regarding insulation resistance testing.
My query is really regarding insulation testing when PAT testing i am experimenting using a Low resistance ohmeter with a Kewtech PAT adaptor

My understanding is that you apply a voltage of 500v to an appliance(reduced to 250v if 500v would damage the appliance) you then get a reading on the tester the higher the better my tester is giving me a greater than 999M Ohms on appliances i have tested within my home.

My main question is what would cause the readings to be low is it just the insulation deteriating or can there be other faults? if so what are they and secondly could i simulate a fault using a length of cable with a lamp on the end to give me a better understanding.

Thanks for your time.

Ricky

You do not need to do this for PAT.

 

[JohnD]

When I used to do PATing, it was almost invariably water appliances (kettles, coffee-makers, tea-urns, steam-irons, washing machines etc) that had leakage. Mostly where the heating element had deteriorated with age, but it can also be due to a water leak inside.

The same things happen with non-portable appliances - in houses, usually immersion heaters, central-heating pumps, boilers, ovens, electric showers.

The other thing is damaged cables, especially flexes on irons or hoovers that are repeatedly rolled up, or where they have been put in a place where they can be walked on, have appliances stood on them, or gnawed by rodents.

Rarely it happens in a device that has overheated, where insulation may have burned off or soot collected on a PCB.

Sometimes, it is said, carbon dust in an electric motor can collect, but I never saw this.

 

[Spark123]

Hello

I am hoping somebody could give me a bit of information regarding insulation resistance testing.
My query is really regarding insulation testing when PAT testing i am experimenting using a Low resistance ohmeter with a Kewtech PAT adaptor

You're wasting your time, an insulation resistance test needs to be done at 500vDC, not with a low resistance ohm meter.

My understanding is that you apply a voltage of 500v to an appliance(reduced to 250v if 500v would damage the appliance) you then get a reading on the tester the higher the better my tester is giving me a greater than 999M Ohms on appliances i have tested within my home.

The 500vDC is required to test appliances supplied at mains voltage, if the device could be damaged then a leakage test as part of the operation test can be carried out.

My main question is what would cause the readings to be low is it just the insulation deteriating or can there be other faults? if so what are they and secondly could i simulate a fault using a length of cable with a lamp on the end to give me a better understanding.

Insulation deteriorating or an internal fault to the appliance can cause a low reading, as can some surge arresters.
I doubt you'll see much with a lamp as the IR test measures Millions of Ohms, a lamp is usually only a few tens of ohms so it will just show as zero Meg.

 

[JohnD]

if the device could be damaged then a leakage test as part of the operation test can be carried out.

did you mean "Can't"?

 

[RF Lighting]

No, it's done by powering the appliance with 230V ac and actually running it, whilst monitoring any leakage the appliance might have, rather than applying 500V dc to the applince before it is acually powered up.

 

[Spark123]

Items such as PCs can be damaged by an IR test at 500v. The leakage test replaces the Insulation Resistance test, it basically powers up the appliance and measures the current flowing in the appliances CPC.
On the Megger PAT 4 this is the last of the tests, prior to this is the earth bond (low current).

 

[Grizzly]

Time to clear up any confusion:

500V Insulation Resistance test
The 'standard' test, but may not be suitable for IT or other electronic appliances, or items containing surge protection.
Line and Neutral conductors are combined, and then 500V dc is applied between the combined live conductors and earth (for Class I) or a test lead attached to an accessible conductive surface on the appliance (Class II). Any current flowing is measured, and so a resistance can be calculated. As this is the resistance between the live parts and you, it should be as high as possible, being measured in millions of ohms (M&#937.

250V Insulation Resistance test
As above, but performed at 250V dc.
Mostly used for such things as extension leads that contain surge protection. The MOVs (Metal Oxide Varistors) that provide the surge protection, usually 3 in number, are connected L-N, L-E & N-E. They usually start conducting around the 300V mark (ish), so a 500V IR test would make them conduct, and so give a false result. The test conducted at 250V would not cause them to conduct, and so give a truer reading of the actual insulation resistance. Again measured in MΩ.

Protective Conductor/Touch Current test (AKA Earth Leakage)
For appliances that could possibly be damaged by the 500V IR test, such as some IT and electronic appliances.
The appliance is energised to normal operating mains voltage, and the current flowing in the earth (Class I) or an accessible conductive surface (Class II, via a test lead) is measured.
This should be a low as possible, being measured in milliamperes (mA).

It should be noted that any appliance that is manufactured to BS EN 60950: Specification for safety of information technology equipment including electrical business equipment should be able to be IR tested at 500V dc.

 

[chalkmycue]

Sparky123

The tester i use is able to test at 250vdc 500vdc and 1000vdc is this instrument suitable or have i got it wrong?

Is there any other way i could set up a fault for test purposes only?

 

[RF Lighting]

Can you give us the make and model of your tester?

 

[chalkmycue]

yeah no problem is a Socket & See DIT400(Digital Insulation/Continuity Meter)

 

[conny]

JohnD wrote,

Sometimes, it is said, carbon dust in an electric motor can collect, but I never saw this.

Believe me, just look inside any DC motor around the commutator end!
Or even in a vacuum cleaner motor. Because they have carbon brushes in them they get 'shaved' each time they pass over the comm segs leading to a build up of carbon. This even happens on 'forced draught' motors which employ an external fan for cooling the motor windings during running. The build up of carbon can soon lead to a low IR due to tracking.

 

[JohnD]

should have said, I never saw a motor cause a PAT fail.

yes, I have seen black dust in old motors.

 

[ericmark]

Grizzly is spot on. However there is another reason for doing the
Protective Conductor/Touch Current test (AKA Earth Leakage)
On items with relays and timers the dead test will not test anything past these devices.
The live test however allows devices not energised in dead test to be tested.

However some timers will not energise within the time that the live test is carried out and with some items some dismantling is required to test or certain per-test procedures need to be followed.

Example when testing fridge or freezers I will unplug as I enter the room then test last in the hope they have warmed up enough so motor is running during test.
If the fridge or freezer has auto-defrost then it really needs testing by service agent as you need to dismantle to fully test.
Washing machines, Tumble driers, and Dish washers should also be tested by service agent because of having timed sequences one would need to test for an extended period which most testers would not allow.

Portable Appliance testing is part of Inspecting and testing of in service equipment and would not include any item over 18Kg unless it includes handles and wheels. However all electrical items do need to be under a system to ensure it is maintained in a safe condition.

The Equipment Register I used had an extra column where the organisation responsible for maintaining/inspecting/testing was entered.

This is what I feel is the problem with allowing semi-skilled personnel to do the first inspecting and testing. Once an electrician has completed the first inspection and logged the tests required then where a simple PAT test can be carried out it seems OK to use semi-skilled labour.

The manager also must accept some responsibility. A fridge test where current measured is 0.01A has been done without the motor running and he should pick this up and ask for a re-test.

All is OK until something goes wrong. One hopes that the earth continuity tests will ensure any fault will open the protective device but portable appliances do get moved around and so no one can guarantee it is always plugged into an earthed socket.

Most accidents are not caused by a single fault. It is normally a combination of a number of faults. Not so long back on this forum a woman's death was reported where a faulty heater was plugged into faulty house wiring in spite of the husband being suspicious of there being a fault. And in such cases the court has to apportion blame.

If the same was to have happened in a factory no one would had got away scot free. As it was in a domestic there was no PAT testing and all parties were able to show they had acted responsibly although not fast enough.

One hopes ones insurance will cover. And items you test will not be part of the chain of events leading to an accident. But read the trade papers and so often one thinks "There by grace of god goes I"!

 

[Spark123]

yeah no problem is a Socket & See DIT400(Digital Insulation/Continuity Meter)

It is an insulation resistance and low resistance ohm meter.
I find using apparatus suited to being used for installations a bit clumsy when doing PAT testing. A proper PAT tester is so much easier!

On a side note, I'd expect the MOVs in a surge protector to start conducting a bit higher than 300v, the AC sine wave peaks at 340v and -340v.