how serious is a line/earth res of 0.91Mohm?

In a recent electrical test they found a resistence from line to earth of 0.91 Mohm on the upstaiors socket circuit in a hosue that I let. The electrician who measured it said is was acceptable and we let the house. Now the paper report has arrived months later and it says "urgent attention" necuase the acceptable limit is 1MOhm

How serious is this 0.91 Mohm line/earth resitence.

The house is protected by (newly installed) RCDs a with RCBOs on each circuit. It was during the installation of these that the resistence was measured.

thanks for any guidance,

Robert

Reply to
RobertL
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From a practical point of view its not going to matter. From a legal point of view it sounds like you have a problem.

Reply to
dennis

Until the 17th edition of the wiring regs came in the lower limit was

0.5 Megohms. However any reading so low does require further investigation.

Such a low readng does indicate a posible problem but it might be a simple thing (or not) to rectify. Just one preliminary question - did the electrician conduct the test with the RCBOs in circuit and their white flying leads connected to earth? If he did this can cause anomalous readings.

Reply to
cynic

That's interesting about the regs, thank you. The house was wired in the 1960s and I have just had the old CU replaced with a modern one. They also improved the earthing.

I don't know how he did the test, but this is a very reputable company and I believe they would do it correctly. Also, the other circuits all show 200MOhm, only this one reading is low.

Robert

Reply to
RobertL

RobertL wibbled on Wednesday 13 January 2010 09:54

On a PIR or EIC, *for LV circuits, eg 230V* it would be a fail under the

17th (I'm guessing it would be a Code 1 on a PIR) but was a pass under the 16th (the limit them being 0.5MOhm, 17th it is 1MOhm) [Reg 612.3.1/Table 61]. This must be tested at 500V DC for normal LV circuits.

That won't trip assuming the 0.93MOhm DC resistance equates to a similar value of AC impedance - it's only leaking about 1/4mA

However, it is a seriously low figure. New wiring will usually go off the scale (> 299 MOhm on my meter) so something is wrong.

I would suggest you investigate it as a priority. Don't bust a gut in this snow if you can't get anyone out, but *do* get it looked at with reasonable expediency. Don't leave it...

You have to look at it pragmatically. It was "legal" under the 16th (but so low, now self respecting sparky would leave it like that).

The question to ask is why is it failing?

a) Damp in an accessory or junction box - OK, not too disasterous.

b) Damaged/degraded cable - could be bad - the cable may fail further and get hot (fire risk).

c) Something else, eg lots of small leakages everywhere adding up.

Regarding b) there's not enough leakage to cause overheating right now, but if you leave it 6 months, that could change, which is why it's important to tend to this.

Reply to
Tim W

You need to identify all the joints, sockets, switches, etc on the circuit. Go around and inspect each one for signs of burning, damp, dust, etc. and clean/replace as required. While you are doing so disconnect and check each cable in the circuit. That should find where the problem is. BTW it could be something as simple as a filter being connected across the circuit so make sure all appliances and extension leads, etc are unplugged for tests.

Reply to
dennis

There is a faint chance your installation might have a non linear resistor surge diverter somewhere. These show different resistance to elevated voltages. Try another test at 250 volts which if showing satisfactory would indicate whether this is the case. Also worth looking for nails or screws being inserted into dry plaster but piercing a buried cable. People hang pictures in the strangest places!

Reply to
cynic

.91 megohms is very very serious indeed.

.91 millohms, is not.

Reply to
The Natural Philosopher

sorry, flip those around.

brain fade got me as I hit the post button.

Reply to
The Natural Philosopher

When I did my PAT certification some time back, the sad thing was that it was all the professional electricians on the course who didn't understand the difference between these two. I think pretty much everyone else did.

Reply to
Andrew Gabriel

The Natural Philosopher wibbled on Wednesday 13 January 2010 13:11

I thought you were quoting loop impedances for a minute ;->

But I would disagree. I don;t think the OP's house is going to blow up tomorrow, but I would certainly want ti get to the bottom of an IR reading of

Reply to
Tim W

Basically you need to...

1 - Identify Final Circuit re route, accessories etc. 2 - Verify *everything* is unplugged & turned off.

Neon & Filters in-circuit will directly affect the result - a missed fluorescent lamp, a hidden neon fused spur for a long disconnected alarm panel etc, bedside clock, plugged in extension lead with neon.

3 - Visually inspect CU end for damage. 4 - Remove all wiring accessories & visually inspect cable & boxes. 5 - Perform IR test at 250V & 500V for each "leg" between wiring accessories. 6 - If all ok, replace wiring accessories.

Watering a plant in front of a socket can cause sufficient water ingress to stuff IR (Insulation Resistance) test results.

A nail or screw is just nicking a cable be it in a wall or in a notch under floorboards can cause a bad IR result, as can a cable knocked against a central heating pipe underfloors - identify which length of cable & correct.

At least they have narrowed it down to one final circuit at relatively low expense.

The figure is too high for a neon, but not certain mains filters and PIR (outside light off upstairs circuit?).

Reply to
js.b1

The value is virtually constant indicating that it's not due to damp or dependent on likely changeable conditions. I therefore think it's not likely to be dangerous. Having a RCBO on the circuit also reduces risk.

The consistent value does sound like a neon in series with a resistor. Given the age of the wiring, I'd be searching for switches and outlets with neon indicators. They will also light under testing if connected between live and earth!!

Reply to
Fredxx

I don't know that the value is constant but it might be. They measured it once and wrote it in their report. All the other values are 300MOhm. They did the measurements as part of the job of upgrading the CU to RCD+RCBOs.

Robert

Reply to
RobertL

I wonder how the testing equipment (megger?) is checked. We've been told that we now have to have our megger certified, and that could cost the price of a new megger. Why should it cost so much to check test gear, and why can't I just check the megger against known certified resistances?

Reply to
Matty F

Matty F wibbled on Thursday 14 January 2010 00:10

It's calibrated with an even more expensive bit of test gear which costs a fortune. But the actual calibration itself should be costing arounf 60 quid

+/-20 - that's for a multifunction meter.

You can (and should) check the megger against a bunch of resistances as a quick check that it hasn't suddenly gone weird on you. There's a product called CalCard that is the size of a credit card with a bunch of pads connected by some low and some high resistances.

But you can make your own.

Reply to
Tim W

We have a Wheatstone bridge with which I could check the resistances. Some new inspector has decided to follow some rules or other that don't really apply to us. The resistences that I measure are always in the range 2M=E2=84=A6 to 200M=E2=84=A6, or a dead short!

Reply to
Matty F

presumably these resistence tests use a Megger because you need the high voltage resistence; the current is not proportional to the voltage. Otherwise I could just stick a current meter in the line (in place of the fuse if there was one) and see if the current is less than 240 uA (240 Volts across 1MOhm). but that would be testing at

240V only which is presumably no good.

Robert

Reply to
RobertL

RobertL wibbled on Thursday 14 January 2010 10:50

That would be better than nothing and could be a quick and dirty way to narrow down some faults (although it is dangerous in one respect - you hve to keep energising and de-energising the circuit between isolating bits of it meaning you are bound to forget one important step eventually)

500V DC is usually the test voltage of choice as it's a safety-margin higher than the 340V peak of a 240V sine - and it will measure nothing but pure resistance, ignoring any capacitive effects of the cables. Although the fact it is easier to make a 500V DC test instrument rather than an AC one may have something to do with it as well...
Reply to
Tim W

Because you need to check that the megger is putting out the correct voltage to do the test properly, not just measuring the resistance right,

Owain

Reply to
Owain

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