Low resistance on microwave oven

[Harry Bloomfield]

It works perfectly. Is there a high probability that the mains cable has a kink and replacing it would be the fix. I understand that no one can be certain, but my other option is to scrap it

Most likely, the person doing the initial test, is at fault, and suggested by the appliance guy - the microwave has no fault at all. Most such appliances, include a mains filter, which normally will show a low resistance during a PAT, testing between line (L or N) and earth. One essential test, is a resistance tested between the earth pin of the plug, and the metal case of the appliance, to ensure it is securely earthed, when plugged in.

When I say that I’m not sure which cables I mean mains to earth or mains to neutral. He tested the circuits as part of a eirc test

That is an entirely different test, to the PAT. It is intended to just test the wiring installation, all appliances, including your microwave should have been unplugged at the start, before carrying out any tests.

 

[ericmark]

Never fails to amaze me how many people think PAT applies to anything electrical. P = Portable, a microwave is not really classed as a portable device.

I know we often called it PAT testing, however as far back as I can remember it was called the inspection and testing of in-service electrical equipment, all electrical installations and equipment should be inspected and tested, but it suits in many cases to split the work into two groups, the inspection and testing of in-service electrical equipment was split into two exams, as there are items with need something special to do the inspection and testing, be it a gas safe certificate, or the keys to a drink's dispenser, so the building manager is responsible for ensuring someone does the inspection and testing, and that whoever does it, has the skill.

And the inspection and testing of the installation, does not normally include any current using equipment, but the building manager can arrange exceptions. For example a hand drier in a toilet would normally be PAT tested, but the logistics of safely disconnecting it to be tested, are far easier with two people, so to arrange for that item to be done when the EICR is done, makes a lot of sense. However, the results will be recorded with the equipment register not the EICR paperwork.

Most electricians do not want to do PAT testing, it is boring, so it is common to get someone through the exam, and let them retest items, once the test parameters are entered into the machine, one only needs to enter the plant number, and the machine shows him/her what to test.

But there is no requirement to use a pat testing machine, one can use low ohm meters, insulation testers, etc, instead, specially when it does not have a plug. Needs more skill, but can be done.

However, one does not always get the expected results. Last week I had this problem, a RCD was tripping, so something was wrong, but my testing failed to find the problem, it was found by disconnecting the faulty circuit. It seems down to a faulty socket, and once I get a replacement, I will take the faulty one apart to find out what it was.

However, it has been proved that it is something with the socket, not the light which was plugged in to the socket. From what you are saying the microwave was not tested in isolation, so the tester does not really have a clue what was faulty.

 

[FrodoOne]

I have an expensive microwave / oven that has failed a pat test on low resistance - im not sure on which cables. I had a domestic repair company look at it and they can find no fault. It works perfectly. Is there a high probability that the mains cable has a kink and replacing it would be the fix. I understand that no one can be certain, but my other option is to scrap it

This statement/question is annoying on several levels.
Firstly, since no acronym should ever be written in any document
unless it is therein explained,
it has not been explained that PAT means "Portable Appliance Testing".

Hence,
"PAT Test" means "Portable Appliance Testing Test".

This is similar to the annoying references to "PIN Numbers" -
which then means "Personal Identification Number Numbers",
a "tautology", at least!
(This was referenced in Post #14)

One must wonder how the device was "tested".
Of course, there should not be any "leakage"/"low resistance"
between any Line or Neutral and Earth connection
but
there will be "leakage" seen between the Line and Neutral on the input to a "Microwave Oven"
which is nominally turned OFF,
because of the "standby electronics" across the Line and Neutral in this condition,
since "microwave ovens" do not actually have an ON-OFF switch.

 

[Rossco107]

Clearly my original post wasn't perfect but I have learn quite a bit from the feedback for which I am grateful. Perhaps a clarification. An electrician was testing the electrical circuit. The circuit showed poor resistance. On the circuit a built in combo microwave / oven / grill was plugged in. When the combo microwave / oven / grill was unplugged the circuit showed appropriate resistance. The combo microwave / oven / grill was not included in other appliance testing as it was assumed to have poor resistance. The circuit with the unit is protected my by an RCD which as never tripped. The unit works perfectly. What I have taken from the feedback is that a unit like this may show lower resistance than the circuit tester expected due to the nature of the unit, without being faulty.

Some responders point out that PAT testing, duplicates the word test - may pet hate is VIN number referring to car VIN plates.

 

[Harry Bloomfield]

What I have taken from the feedback is that a unit like this may show lower resistance than the circuit tester expected due to the nature of the unit, without being faulty.

You would seem to have got it!

Your major concern ought to be whether the case of the unit, has a good connection to the earth pin of the plug.

 

[ebee]

An electrician was testing the electrical circuit. The circuit showed poor resistance. On the circuit a built in combo microwave / oven / grill was plugged in. When the combo microwave / oven / grill was unplugged the circuit showed appropriate resistance.

I find it rather strange that An "Electrician" had done thus!

 

[FrodoOne]

Clearly my original post wasn't perfect but I have learn quite a bit from the feedback for which I am grateful. Perhaps a clarification. An electrician was testing the electrical circuit. The circuit showed poor resistance. On the circuit a built in combo microwave / oven / grill was plugged in. When the combo microwave / oven / grill was unplugged the circuit showed appropriate resistance.

I strongly doubt that "the circuit showed poor resistance".
Perhaps it showed "poor insulation resistance".
i.e. Leakage.

 

[Harry Bloomfield]

I strongly doubt that "the circuit showed poor resistance".
Perhaps it showed "poor insulation resistance".

The OP perhaps misunderstood what was said to him. 'Send 3 and 4 pence we are going to a dance'.

 

[FrodoOne]

The OP perhaps misunderstood what was said to him. 'Send 3 and 4 pence we are going to a dance'.

Or "Send us reinforcements, we are going to advance."

 

[Risteard]

Most likely, the person doing the initial test, is at fault, and suggested by the appliance guy - the microwave has no fault at all. Most such appliances, include a mains filter, which normally will show a low resistance during a PAT, testing between line (L or N) and earth. One essential test, is a resistance tested between the earth pin of the plug, and the metal case of the appliance, to ensure it is securely earthed, when plugged in.


That is an entirely different test, to the PAT. It is intended to just test the wiring installation, all appliances, including your microwave should have been unplugged at the start, before carrying out any tests.

Neutral isn't line - it's live.

 

[ebee]

Neutral isn't line - it's live.

Probably just a typo.

Lots of folk say Live instead of Line so perhaps this one was this, but in reverse?

 

[Risteard]

Probably just a typo.

Lots of folk say Live instead of Line so perhaps this one was this, but in reverse?

Not a typo - many do not understand the difference between line and live and use them incorrectly. Personally I prefer the term "phase" to "line".

 

[ebee]

"testing between line (L or N) and earth." was the bit I was reffering to s perhaps a typo.
Should be - testing between live (Line or N) and earth. Then it makes sense
The only time N is not considered Live if it is N & E combined and that is only by a convention.

Yes I agree that phase is better than line quite often too

 

[ericmark]

Of course, there should not be any "leakage"/"low resistance"
between any Line or Neutral and Earth connection

Not quite the same thing, leakage can occur due to capacitive or inductive linking, resistance is DC, so there is no capacitive or inductive linking, this only happens with AC, and with AC it is called impedance not resistance.

Also, any surge protection device is likely to conduct at 500 volts, so if a microwave has some filter built into it to stop the microwaves being transmitted back down the supply lead, which most will have, then it is likely that any insulation tester set to 500 volts DC will show the low resistance due to the voltage being used.

So an electrician should check for any filtered sockets, any sockets with USB outlets, or any sockets with neon indicators and ensure nothing is plugged in before doing a 500 volt insulation test. If it is not possible to remove the items, then maybe a 250 volt test can be done instead, with of course a note on the report to say why a 250 volt test only was done.

It seems from the description and error was made, and the electrician left the microwave plugged in when making the test, one hopes his test did not damage the microwave, but the fact the microwave showed a short circuit with a 500 volt test shows the unit is in good order and has good quality surge protection device fitted, only if it showed a short circuit at 250 volts would there be a cause for thinking the microwave was faulty.

I was going to say no cause for alarm, but there is a cause for alarm, the electrician has made an error which may have damaged the microwaves filters. But unlikely this will cause any danger, other than maybe some radio interference due to the damage he has done.

I find it rather strange that An "Electrician" had done thus!

I suppose we have all made mistakes, pobody is nerfect, but to try to cover up ones mistakes by blaming a microwave rather ones own error is clearly wrong.

The problem is once one error is made, it puts into question the guys' ability to do the work, to miss unplugging a microwave is excusable, to say the microwave was at fault is not, and raises questions as to if any other errors have been made.

 

[ebee]

500vThe problem is once one error is made, it puts into question the guys' ability to do the work, to miss unplugging a microwave is excusable, to say the microwave was at fault is not, and raises questions as to if any other errors have been made.

Well excusable maybe or maybe not (I would err to towards not mostly) , but I do agree whole heartedly with "to say the microwave was at fault is not, and raises questions as to if any other errors have been made." I totally agree Eric.

Go around and have a good look and try not to miss anything.
Test at 250V initially,
if all seems OK then go to test at 500V.

500V is the "proper test" and must always be done with new works you done yourself (you should know what you`ve done and where it is) , 250V is not ideal but better than not testing at all if you not 100% sure that everything is disconnected.