Volt drop problem

[JohnW2]

I've already agreed with this.

Fair enough. As I said, the test he did must have been a 'loop impedance test', but are you saying that his meter shows Ze and PFC separately (only one at a time), and that he only saw the PFC and would have had (but did not bother to) press/do something to display the actual Ze? With my Fluke, I'm used to being presented with both, as in (just done) ...

... which obviously implies a voltage at the time of about 242 V (425 x 0.57)

I agree with all of that... Except that unless I have got it wrong, ignoring the Wylex which is effectively a junction box (shown as orange circles) with the power turned off with all of the 2.5mm² & 4mm² simply connecting the 2 sockets together and testing like thiswith a short at one socket and meter at the other must, by definition, find that high resistance? That's the test I've done, in both directions.

Yes, on the basis of the other things you have reported, I would expect that test to show a 'high' resistance (seemingly probably about 115Ω ), in both 'directions'. Are you saying that you only got very low readings (say, <1Ω )?

Kind Regards, John

 

[SUNRAY]

Fair enough. As I said, the test he did must have been a 'loop impedance test', but are you saying that his meter shows Ze and PFC separately (only one at a time), and that he only saw the PFC and would have had (but did not bother to) press/do something to display the actual Ze? With my Fluke, I'm used to being presented with both, as in (just done) ...

I don't know the KEW product, My Robins show one or the other, switchable and I believe 3 ranges each.

View attachment 281458

... which obviously implies a voltage at the time of about 242 V (425 x 0.57)

Yes, on the basis of the other things you have reported, I would expect that test to show a 'high' resistance (seemingly probably about 115Ω ), in both 'directions'. Are you saying that you only got very low readings (say, <1Ω )?

Kind Regards, John

Yes that's what I've been saying all the way thorugh <1Ω and why I wrote:

... all I can think is something is breaking down under heavy current but not under tester current.

 

[SUNRAY]

Of course curiosity got the better of me and I had to get it out...

My Robin KMP4120DL has switchable ranges of 20Ω, 200Ω, 2KΩ, 200A, 2KA & 20KA and just a single display
At home I currently have 0.34Ω & unusually high 241-244V (fluctuating) and 588-597A
Those figures don't quite tally in my head, PSC seems a little low.

 

[JohnW2]

I don't know the KEW product, My Robins show one or the other, switchable and I believe 3 ranges each.

Fair enough. As I said, if it's displaying a PFC, it must be undertaking a loop impedance test so, as before, I presume what you're saying is that your colleague did n press/do whatever would be necessary to get it to display the loop impedance?

Yes that's what I've been saying all the way thorugh <1Ω

In that case, I agree that it seems essentially 'crazy'

and why I wrote:

... all I can think is something is breaking down under heavy current but not under tester current.

Maybe, but I'm not sure what could be 'breaking down', particularly in a fashion which was resulting in an increase in resistance.

I'm tempted to suggest that yo probably really need to 'put your foot sown' since it sounds as if you are being asked to diagnose this fault 'with your hands tied behind your back'. I imagine that if you could get at the switch-fuse and disconnect the cables, you would probably be able to get to the bottom of this in five minutes.

Kind Regards, John

 

[SUNRAY]

Fair enough. As I said, if it's displaying a PFC, it must be undertaking a loop impedance test so, as before, I presume what you're saying is that your colleague did n press/do whatever would be necessary to get it to display the loop impedance?

In that case, I agree that it seems essentially 'crazy'

Maybe, but I'm not sure what could be 'breaking down', particularly in a fashion which was resulting in an increase in resistance.

I'm tempted to suggest that yo probably really need to 'put your foot sown' since it sounds as if you are being asked to diagnose this fault 'with your hands tied behind your back'. I imagine that if you could get at the switch-fuse and disconnect the cables, you would probably be able to get to the bottom of this in five minutes.

Kind Regards, John

The operational restrictions of the complex are a headache, this is the third job I've done there so it's not a surprise. There are only 3 people in the company with appropriate security clearance; MD, colleague and me as a subby.

The site has a massive security presence and the building this is in is a big old house, with a long and arduous walk from the fusebox to bedroom socket.

They have no qualms about the cost of aborted/shortened visits, phones and cameras are strictly forbidden and locked away in steel lockers while on site. Diagnosing/repairing electrics is low on their list unless it impacts on the business.

Yes it is a wierd fault nothing about itmakes sense, I went in believing it was a broken socket - physical damage.

 

[JohnW2]

The operational restrictions of the complex are a headache, this is the third job I've done there so it's not a surprise. There are only 3 people in the company with appropriate security clearance; MD, colleague and me as a subby. The site has a massive security presence and the building this is in is a big old house,

Fair enough. I occasionally have comparable problems with 'security', albeit not usually in relation to 'physical access' security. When that happens to a significant extent, I usually keep reminding them of my 'hourly rate' and point out that I have therefore already totted up hundreds, if not thousands, of pounds to put on my bill for a job that might well have cost only £50 if it had not been for their 'security' systems

with a long and arduous walk from the fusebox to bedroom socket.

I'm a bit confused. What is this 'long and arduous walk' - I thought the socket in question was about 4 metres from the 'fusebox'?

Kind Regards, John

 

[SUNRAY]

I'm a bit confused. What is this 'long and arduous walk' - I thought the socket in question was about 4 metres from the 'fusebox'?

Kind Regards, John

Yes it is, it is pretty much the other side of the wall but there is no internal access (think of it as a pair of semidetatched houses) one side is part of the high security part of the complex and access is actually from a different road and via 3 security positions.

It turns out my colleagues KEW MFT is in for calibration and has been using uncalibrated Robin testers, virtually the same as mine, so PFC and LI are not simultaneousely displayed.

Anyway I've just got home from there and signing in we had to state the reason for visit, the security guard was quite chatty and mentioned the socket in the office used to have the same problem and the disused white cable (the 4mm² in this thread) was used to fix it and pointed to the old cable hanging under the counter terminated with choc bloc and plastic tape.
We both tested for PFC & LI but each time the button pushed a different reading between ¼ & 2A or 280 & 1700Ω. consistant with the socket in the bedroom.
In the high security side security office back room my colleague found a junction box behind filing cabinets, and where the cable goes through the wall to the bedroom, with the original tee. we had to replace the short piece of cable and isolate the redundant piece which were both neutrals burnt.

 

[JohnW2]

Yes it is, it is pretty much the other side of the wall but there is no internal access (think of it as a pair of semidetatched houses) one side is part of the high security part of the complex and access is actually from a different road and via 3 security positions.

Ah, OK - thanks for explaining.

I presume you already have done so, but it certainly sounds as if you should impress on them how much easier (and cheaper!) it would be if you had 'easy access' to that switch-fuse!

As before, Good Luck!

Kind Regards, John

 

[jj4091]

As John W2 has already mentioned surely from a health & safety point of view the switched fuse appears not to be in a safe place, could you suggest relocating to the easy access side of the wall as part of the job

 

[JohnW2]

As John W2 has already mentioned surely from a health & safety point of view the switched fuse appears not to be in a safe place, could you suggest relocating to the easy access side of the wall as part of the job

Good idea - but that relocating would obviously require access to the 'difficult to access' location

Kind Regards, John

 

[jj4091]

Good idea - but that relocating would obviously require access to the 'difficult to access' location

Kind Regards, John

So will fixing the fault

 

[JohnW2]

So will fixing the fault

Very True. In fact, it would not surprise me if the act of 'relocating the switched fuse' itself 'fixed the fault' !

However, my point was that, although it could make life easier in the future (and would be preferable, H&S-wise), in the immediate sense, re-locating the 'switched fuse' and 'fixing the problem' (if different!) would involve the same practical (access) problems.

Kind Regards, John

 

[SUNRAY]

Apologies I managed to click on 'post' before I finished then started editing post ~22 and had dinner and a phone call yada yada yada before I finished edit and posted.

The fusebox is one in a horrible added to over the years installation with something like a dozen more on 3 (one per phase) Henley blocks. Yes it's a mess but I can think of a number of relatively similar situations,
Additionally I can think of relatively similar radials without quick access to the fusebox, such as a garage at the end of the garden with a socket as a spur on the house ring.

Incidentally the R1 + R2 test results of the 2.5mm² leg before finding the fault: L-N 0.08Ω , L-E 0.14Ω , N-E 0.15Ω so no apparent fault, the accurate reading could be 144.9 & 145.1. However terrible and inconsistent PFC & LI which is competely expected following the kettle test.
From the 80mΩ L-N reading I'll make the calculation there is between 5.06 & 5.74m
From the 140mΩ L-E reading I'll make the calculation there is between 5.29 & 5.68m of 2.5 + 1.0mm²

After replacing the damaged cable (<½m long) with 2.5+1.5mm² T&E: L-N 0.08Ω , L-E 0.14Ω , N-E 0.14Ω and 0.31Ω LI & PFC 682A

 

[SUNRAY]

Over a beer tonight my colleague described the terminals in the junction box, they sounded similar to those found in a motor with links:

He abandoned it and instead joined all the wires using Wagos as that used by the neutral was damaged.

 

[JohnW2]

Over a beer tonight my colleague described the terminals in the junction box, they sounded similar to those found in a motor with links: ... He abandoned it and instead joined all the wires using Wagos as that used by the neutral was damaged.

I was meaning to ask when I first saw that 'explanation', although you have resolved the problem, what about the reason why the neutral got damaged (I think you said 'burnt') in the first place - are you just blaming 'loose connections', or what?

Kind Regards, John