No power to immersion

[HappyHippyDad]

Morning all.

I have just been to a house where there is no power to the 13A FCU feeding the immersion.

I believe I have found the corresponding MCB in the CU although it is difficult to be sure as there is only continuity on the CPC, it is labelled water heater. There is no continuity from the CPC in the FCU to any other CPC's in the CU which also makes me think I have located the correct MCB.

It has not been working for around 6 months. The tenants say no work has been carried out and it just stopped working one day.

It does seem odd that there is a completely clean break between line's and neutral's (L-L >999Mohms, N-N >999Mohms) of the cable and the CPC is intact, barring any parallel paths (unlikely).

I have searched everywhere for a DP switch or timer switch but cannot find one. The tenants say there isn't one and the landlord is unsure.

Am I missing something obvious? The reason I ask is because I will be putting a new circuit in (easier than taking carpets/floorboards up to find fault) but it just seems like there must be another switch somewhere and that's a lot of disruption for the sake of flipping a switch!

 

[Andy78]

Very common in older houses round here to have an old DP switch with neon mounted in the hallway or in the understairs cupboard. Often these get left in when rewired leaving two points of isolation. I've also seen them left connected but stuffed under floors after being moved off airing cupboard walls.

 

[littlespark]

They’ve had hot water as there’s a boiler(?)
Why do they need the immersion now? (Not needed it for 6 months) do they think corona is going to shut off their gas supply?

my bet (without further info) is original kitchen dp switch has been replaced with e7 or whatever controller and the connections have melted.

I’ve never used an FCU on an immersion, just a dp switch as it’s protected at CU at 16A
Usually

 

[pc1966]

My kitchen had a DP switch for the immersion heater, nice and obvious and labelled as well. But when that was no longer needed due to a combi boiler going in I replaced it with a 13A socket. You always need an extra socket...

Now if it were easier to do, etc, I would have swapped it with the one for the fridge so it was on a separate 16A spur instead of the main ring, but cables all go through a false ceiling and the work seemed far too much to bother with at the time. Cobbler's shoes, etc.
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Also replaced the FCU that fed the time switch for the old boiler with a 13A socket, but sadly the old boiler is still in there as they built the kitchen around it and I can't remove it as a result
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I’ve never used an FCU on an immersion, just a dp switch as it’s protected at CU at 16A
Usually

I think some just use the FCU as a handy point for terminating 1.5mm silicone rubber heat-resisting wire.

 

[pc1966]

Just though of a way to help you locate the elusive DP switch. In the telecoms world you can get a Time Domain Reflectometer to do this, but that is a rare and pricey bit of kit. However, many sparks will have a multimeter that has a capacitance range, so why not try this:

I measured a 95cm off-cut of Prysmian 2.5mm T&E and found it measured 115pF L-E and 110pF N-E, so that is averaging around 118pF/m. However T&E has no specification or control on capacitance so you could get very different answers, but it should be fairly constant for a given length of cable, so:

First accurately measure the CPC resistance with one end disconnected and if you know the earth CSA then you can estimate the overall cable length based on that value (accepting you might have terminal resistance increasing it and/or parallel earth paths reducing it).

Next measure the capacitance from L to E at each end. The ratio of these capacitances should be roughly the ratio of cable lengths, and from step 1 you then get a rough idea of the distance from each end to the break.

It might be your meter struggles to measure below 1nF = 1000pF which would limit you to around 8.5m, or it might be it picks up too much noise giving crazy readings so you have to power off the whole house, but it is worth a try.

Yes, it is not that accurate and it would not take in to account whatever bizarre route the installer took, but it would give you some sort of upper bound on the distance where any switch could be located.
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Just to add, if you lack low capacitance range on your meter then you could disconnect both L & N to measure capacitance L+N -> E

I get 202pF/m for my sample of 2.5mm T&E (not quite double but close) so the 1nF limit for some meters would be about 5m of cable.
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OK tried this with my Fluke 179 but it has 1nF resolution, which is pretty much not going to give any usable information.

However another thread was discussing multimeters and the Testo 760-1 was mentioned, the instruction book says 0.01nF (i.e. 10pF) for around £62 and its higher spec brother 760-2 has 1pF resolution for the tolerable price of around £104 which ought to work well here.

Testo 760-1 Digital Multimeter

Testo 760-1 Digital Multimeter The Testo 760-1 Digital Multimeter is a versatile multimeter that places great emphasis on usability in addition to measuring all regular electrical variables including AC/DC voltage, AC/DC current, resistance, capacitance, and frequency. Diode and continuity...

www.test-meter.co.uk

Testo 760-2 True RMS Digital Multimeter

Testo 760-2 True RMS Digital Multimeter With impressive ranges over a multitude of different variables, the Testo 760-2 True RMS Digital Multimeter is a great instrument for electrical inspection and assessment. In addition to measuring standard multimeter values such as AC/DC voltage, AC/DC...

www.test-meter.co.uk

 

[marconi]

£40......

[£39.14] NOYAFA® NF-816 Wire Tracker Underground Cable Detector for locating the buried wires - https://www.lightinthebox.com/en/p/noyafa-nf-816-wire-tracker-underground-cable-detector-for-locating-the-buried-wires_p7290847.html?currency=GBP&litb_from=paid_adwords_shopping&sku=1_737&country_code=gb&utm_source=google_shopping&utm_medium=cpc&adword_mt=&adword_ct=411874581765&adword_kw=&adword_pos=&adword_pl=&adword_net=u&adword_tar=&adw_src_id=3516490533_8919533455_93519943161_pla-863009657701&gclid=CjwKCAjwg6b0BRBMEiwANd1_SCysrN3rxL5LWtqA286lwhzCwRmTVDMYao_Y-Ff-CR7N1MRxQpxlzhoCZvsQAvD_BwE

 

[Mike Johnson]

And the next version up: Testo 760-3 True RMS Digital Multimeter - https://www.test-meter.co.uk/testo-760-3-true-rms-digital-multimeter/

I have been using the 760-1 for about a year now and can't fault it in any way, the problem being if it ever goes missing and I go back to a multi position dial meter I am sure I will rely on the auto setting and blow it to smithereens.

 

[pc1966]

£40......

[£39.14] NOYAFA® NF-816 Wire Tracker Underground Cable Detector for locating the buried wires - https://www.lightinthebox.com/en/p/noyafa-nf-816-wire-tracker-underground-cable-detector-for-locating-the-buried-wires_p7290847.html

Have you tried one?

Certainly that is a lot cheaper than the others I have seen advertised and makes it tempting to get one.

 

[marconi]

Have you tried one?

Certainly that is a lot cheaper than the others I have seen advertised and makes it tempting to get one.

No but I thought for the price worth a punt. Spending £40 to avoid spending on a new cable run and sparky time plus perhaps no remedial decoration ........

 

[pc1966]

Very true. I see it listed on Amazon as well but much higher price:
View: https://www.amazon.co.uk/NF-816-C-Underground-Sprinkler-Electrical-Telephone/dp/B00F5WA6QQ

Mixed reviews. One complainng it did not work with SWA cable!

 

[marconi]

Very true. I see it listed on Amazon as well but much higher price:
View: https://www.amazon.co.uk/NF-816-C-Underground-Sprinkler-Electrical-Telephone/dp/B00F5WA6QQ

Mixed reviews. One complainng it did not work with SWA cable!

 

[pc1966]

I have been using the 760-1 for about a year now and can't fault it in any way, the problem being if it ever goes missing and I go back to a multi position dial meter I am sure I will rely on the auto setting and blow it to smithereens.

If you have your 760-1 to hand, and some T&E cable lying around, can you have a go at L-E capacitance measurement, etc, just to see if it gives any usable results here?

 

[Mike Johnson]

I have the 760-1 to hand, but no T&E.

 

[pc1966]

I have the 760-1 to hand, but no T&E.

Maybe try with an IEC lead or similar?

A 2m "clover leaf" style cable here for a laptop supply measures around 180pF conductor to conductor on my capacitance meter, just be interesting to see if your multimeter can show that at all.

 

[Lucien Nunes]

I also often use capacitance to roughly locate an open in a cable that has quirks and interconnections that will give confusing results on a TDR plot. Simply identifying which end of a cable has an open pin on a moulded connector is often all that is required, in which case the bad end will have near zero stray capacitance and the actual reading at the other end doesn't matter. In reality I probably use my capacitance meter more for that kind of thing than measuring capacitors. Capacitance measurement works well on T+E, but the results will vary according to how the CPC is connected and to a lesser extent how it is bundled or in contact with earthed stuff. Probably best to measure between one or both live cores and the CPC.

I had to create a Heath-Robinson TDR setup in the middle of nowhere, when I had access to a computer and a monitor but no test gear. A TDR set sends out pulses and times the delay before reflections are received that indicate open- and short-circuits by their abrupt changes in characteristic impedance. I used the computer and monitor to do the same thing. I stripped a section in the middle of the monitor's RGB cable, broke into the green channel and paralleled a pair of very short test leads. By displaying a clear vertical bar on the computer and measuring the distance on-screen between the true image and the ghost with the test leads connected to each end in turn, I could calculate what fraction of the length lay between each end and the fault (It was exactly 100m if my memory is correct. The cable was defective in manufacture and there was a feeding-in splice in the core that should never have been present in the finished product)

 

[DPG]

I also often use capacitance to roughly locate an open in a cable that has quirks and interconnections that will give confusing results on a TDR plot. Simply identifying which end of a cable has an open pin on a moulded connector is often all that is required, in which case the bad end will have near zero stray capacitance and the actual reading at the other end doesn't matter. In reality I probably use my capacitance meter more for that kind of thing than measuring capacitors. Capacitance measurement works well on T+E, but the results will vary according to how the CPC is connected and to a lesser extent how it is bundled or in contact with earthed stuff. Probably best to measure between one or both live cores and the CPC.

I had to create a Heath-Robinson TDR setup in the middle of nowhere, when I had access to a computer and a monitor but no test gear. A TDR set sends out pulses and times the delay before reflections are received that indicate open- and short-circuits by their abrupt changes in characteristic impedance. I used the computer and monitor to do the same thing. I stripped a section in the middle of the monitor's RGB cable, broke into the green channel and paralleled a pair of very short test leads. By displaying a clear vertical bar on the computer and measuring the distance on-screen between the true image and the ghost with the test leads connected to each end in turn, I could calculate what fraction of the length lay between each end and the fault (It was exactly 100m if my memory is correct. The cable was defective in manufacture and there was a feeding-in splice in the core that should never have been present in the finished product)

That's bloody creative is that.