Electrical resistance

[Darren Forward]

Hi with the power turned off,

What should the DC resistance between L and N be.

I have 11 Ohms.

Many thanks

Darren

[Stefek Zaba]

I should hope so...

Think about it.

If you've properly disconnected all loads on the relevant circuit - it should be 'very high'; multiple megaohms. If there's, for example, one lightbulb across the L and N - pretty damn low (well below the resistance you might calculate for the current it draws when lit - cold resistance is up to 10 times lower than resistance when operating).

Super. Now put away that meter before you do it (or yourself) any damage.

[Darren Forward]

Thanks for that, There's nothing connected across it apart from the connection to the CU.

Thanks Darren

[John Rumm]

And have you opened the micro circuit breaker for the circuit at the CU as well as just turning off the main switch? If not you will be measuring the resistance of other circuits on the CU.

[Darren Forward]

That explained it I pulled all the fuses and got good results.

Thanks

Darren

[John Rumm]

Jolly good...

If you don't mind me asking, what exactly were you trying to find out in the first place?

[Tim S]

Insulation test?

To the OP: If so, it ought to be in the megohm range minimum to pass. Don't know what the current required figure is exactly (2megohms come to mind but I've seen 0.5 M ohm quoted too??).

But testing with a regular multimeter only give a rough check and won't show up failing insulation - for that you need a tester that applies a high voltage (500V or something in that order).

Mind you, if your multimeter says, for example, 100k ohms, I'd be turning the circuits off pretty quick(!)

Here's a random link to Megger's testers for interest,

[Dave Plowman (News)]

You're hardly doing much of an insulation test with all the fuses removed.

[John Rumm]

I always thought it was safest to disconnect the circuit from the CU before doing an insulation test on it... let alone take out the fuse / MCB!

[Owain]

"Darren Forward" wrote | Hi with the power turned off, | What should the DC resistance between L and N be. | I have 11 Ohms.

You have about 20A or 4.8kW of load somewhere on the circuit.

Owain

[Lurch]

On Tue, 01 Feb 2005 20:13:47 +0000 (GMT), "Dave Plowman (News)" strung together this:

You are, you're meant to remove all circuits from the DB and test them individually.

[Dave Plowman (News)]

Well, yes, but not across the input to the CU as the OP was doing. All that does is test the insulation of the CU and tails.

Downstream of the fuses, yes.

[Stephen Dawson]

Not strictly correct. You can test the whole installtion in one go if required, and if the resistnce is above 2MR then no further test is required. If less than 2MR than individual circuits should be checked and recored and a value of 0.5MR and above are a pass.

Regards

Steve Dawson

[Lurch]

On Tue, 01 Feb 2005 23:01:29 +0000 (GMT), "Dave Plowman (News)" strung together this:

Ah, missed that bit. That's not the way to do it unless you know what you're doing.