Testing issues !

[Soulsurfer]

long story short, Pre-rip out test really old install, TN-S,16mm tails, 6mm main earth !
Ze was 0.22 ohms and 1.2kA fault current.

DNO been up this morning to connect new tails 25mm I supplied through switch fuse and 16mm earth. Fit board new, go to check Ze and get 0.00 ohms ! PEFC 10.0kA & PSC 10kA with light showing warning red triangle on touch metal plate on my fluke MFT ! Think I'm super close to a DNO supply transformer like 25 metres away max so would that get the Ze that low ?

 

[Soulsurfer]

long story short, Pre-rip out test really old install, TN-S,16mm tails, 6mm main earth !
Ze was 0.22 ohms and 1.2kA fault current.

DNO been up this morning to connect new tails 25mm I supplied through switch fuse and 16mm earth. Fit board new, go to check Ze and get 0.00 ohms ! PEFC 10.0kA & PSC 10kA with light showing warning red triangle on touch metal plate on my fluke MFT ! Think I'm super close to a DNO supply transformer like 25 metres away max so would that get the Ze that low ?

Just went for a walk and found big transformer 20 metres away max as guessed & new batteries, leads, etc on mft but still 0.00
234 incoming so looking at it 234/0.00 is just the same : overflow on calculator or high estimate Fault current on tester ! Also used a 20kOhm resistor I have to check meter itself and it measured 20K spot on ! ...

 

[westward10]

How come you originally got 0.22.

 

[The Ghost]

This is always a problem by sub-stations and yes you would easily get 0.00. My meter reads to 3 digits, but it is qualified to within 6 digits in the manual so 0.002 could be 0.008 or.. less. Of course you cannot divide by 0 as you wind up with the original number. Also you must divide by 230 nominal voltage and multiply that by Cmin which comes out at 218.5 V./---. See Guidance Note 3. So if you can get a 3 digit ze reading then you may be able to do PFC calc. However that wont help as you will probably end up with massive Ka PFC i.e. .005=44Ka beyond any fuse capacity normally used domestically. As next year the 18th is bringing in 25mm tails/Main earth (I think) this is a problem that is going to get worse, not better.

 

[Wilko]

Hi SS - yikes 0.00 ohms - If they've cabled it in 25/16 and it's say 30m effective, the OSG says about 0.056 ohms I think (noting bottle of red next to calc). To get 0.00 ohms result you'd need to be measuring a local short circuit - and you could get one of those if you were measuring N to E on TNCS perhaps? Quick check on polarity resulting from their connection of your new tails? Long shot, but it's all I got, sorry.

Edit - just read V's reply

 

[Soulsurfer]

How come you originally got 0.22.

I'm wondering if batteries in mft were getting tired as was down to 6.5V as opposed to around 9V in battery compartment before it runs through internal electronics etc.. some have posted erraneous results with low batteries. I tested some pipework with an R2 lead and got a high reading too then consistent lower readings afterwards, so wondered if sometimes mft behaves daft !

 

[The Ghost]

Maybe you should check in your manual the +- tolerances of measurement on Loop. i.e. mine is within 6 digits whats yours? And did they stick with TNS or change to TN-C-S?

 

[Risteard]

Of course you cannot divide by 0 as you wind up with the original number.

That's if you divide by one. If you divide by zero the answer is infinity.

 

[westward10]

Old maths problem, what is 0÷0 is it 1 or 0.

 

[The Ghost]

Dohh!

 

[Wilko]

Oh dear - 0/0 - I don't think I ever really got it.
Anything X Zero = Zero , seems ok ... but then Zero/Zero = Anything it feels like? Does my head in. More red needed.

 

[Deleted member 26818]

To obtain a usable reading, you will have to introduce a known resistance.
Get a drum of cable, measure the resistance then connect drum to one of the conductors and then measure the loop impedance.
Deduct the known resistance and you should have two values with which to calculate PEFC and PSCC.

 

[Soulsurfer]

Maybe you should check in your manual the +- tolerances of measurement on Loop. i.e. mine is within 6 digits whats yours? And did they stick with TNS or change to TN-C-S?

0.001 on loop & 15 digits spec.
Still a TNS system too.

 

[The Ghost]

Do you have L-PE and L-N switch on your test. And I always ask what the DNO reads on Cut out change for Ze. Maybe you could still ask them if they recorded it?

 

[Soulsurfer]

Do you have L-PE and L-N switch on your test. And I always ask what the DNO reads on Cut out change for Ze. Maybe you could still ask them if they recorded it?

I do have those switchovers on the MFT Vortigern yes, & im hoping they did a test but doubt it ! Wondering if worth borrowing a Megger or similar and trying again !

 

[Silly Sausage]

Oh dear - 0/0 - I don't think I ever really got it.
Anything X Zero = Zero , seems ok ... but then Zero/Zero = Anything it feels like? Does my head in. More red needed.

Cue Numberphile! The boys will answer all your Maths problems.

0/0 at 9:40, but you need to watch the preamble. I love it!

 

[Soulsurfer]

Cue Numberphile! The boys will answer all your Maths problems.

0/0 at 9:40, but you need to watch the preamble. I love it!

Oh my, I just sat and watched the whole lot of that with my 4th Fosters and understood it and even really enjoyed it !

 

[The Ghost]

worth borrowing a Megger

Well I guess it certainly wont hurt you have to get a figure one way or another. But if your meter goes to 3 digits, you should get a measurement. I would phone the tech dept for your MFT and ask them what you can do, usually they are brilliant at helping out.

 

[Soulsurfer]

Well I guess it certainly wont hurt you have to get a figure one way or another. But if your meter goes to 3 digits, you should get a measurement. I would phone the tech dept for your MFT and ask them what you can do, usually they are brilliant at helping out.

Are you a mind reader Vort ? Just had a call back from Erik in Netherlands (Fluke) he's saying always reset the zero / null memory when that happens as very likely that it's calculated extra resistance to deduct from what would already be a very low result. Makes sense as I sometimes have zeroed again between tests or after an on / off to be certain it's nulled the leads and is ready! Could well be operator error !
Also said always use the zero adaptor and never the probes touched together as because of far less surface area it will throw up silly small resistances and then you hit zero again to drop value and straight off you have a further deducted value ! He mentioned leads but I've tried a few so should be fine but again batteries on below self test should be careful below 8V as can throw up all kinds of results. Going to need shares in Duracell.
Old Megger 1553 wasn't so sensitive ! Prob my fault with zeroing though. Thanks to all of you guys

 

[The Ghost]

Excellent, have you now got a Ze? I think thats a bankable thought when oddities like that happen.