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Old 3rd Oct 2018, 7:07 pm   #41
ColinB
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

You've got some over current protection in the circuit, as well as the 40A rated RCD....? Or at least, I hope you have!

Nice build though, could be interesting to see if the voltage changes as load is applied, but that close to the sub I bet it won't
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Old 3rd Oct 2018, 7:49 pm   #42
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Yes, good point, the 40A rating of the RCD is not actually over current protection this is simply the rated current it is capable of passing.
A normal RCD does not have any over current element fitted, RCBOs do.
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Old 5th Oct 2018, 6:20 pm   #43
G6Tanuki
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Quote:
Originally Posted by dave walsh View Post
I didn't know those LED indicators existed Tanuki That's a very neat holder you've put together!
BTW is your Atavar related to Quatermass by any chance?
they are rather neat little displays: not spectacularly accurate [1% accuracy - meaning the last digit counts in twos not ones] and they have a significant degree of lag/hysteresis [the display only changes every couple of seconds - which I see as a benefit because it's easier to read than one where the last digit's continuously jittering] but for 2 I'm not complaining!

And yes, spot-on with the Quatermass reference!

Quote:
Originally Posted by ColinB View Post
You've got some over current protection in the circuit, as well as the 40A rated RCD....? Or at least, I hope you have!

Nice build though, could be interesting to see if the voltage changes as load is applied, but that close to the sub I bet it won't
Sorry, it's a 40A RCBO not just a RCD.

Watching the display when it's showing 248V, if I then turn every ring and the oven on on the cooker [which should draw about 13 Kilowatts - this is the biggest load I can create] I drop something between 4 and 6 Volts.

I like low supply-impedances.
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Old 5th Oct 2018, 9:28 pm   #44
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Somewhere about a tenth of an Ohm! You're drawing about 52A and losing about 5V along the way, which suggests about 0.1Ω as far as where the circuit to the voltmeter diverges from the cooker supply.
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Old 6th Oct 2018, 7:47 am   #45
G6Tanuki
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

The circuit splits just after the meter: the cooker (along with lighting and ring-main outlets) is on one consumer-unit, the voltmeter's on the other one (which serves all the radial circuits and outdoor loads).

I know I've got a good low loop-resistance: following some repeated issues with trees last year SSE (the DNO round here) installed an underground cable from the substation to the pole opposite my house - a nice new length of 3-phase SWA, about as thick as my arm. They are going to return sometime soon to replace the current separate-wires-on-insulators overhead span into the house with an ABC [Aerial Bundled Conductor] because the separate-wires arrangement is a bit old. I'm not taking a 3-phase supply (they charge extra for that) but it's nice to know that it'd be easy to provision if I did want it.

if I did go 3-phase then I could have one little LED-voltmeter for each phase! (they're available in red, green, blue, orange). I think three of them would *just* fit side by side in a double-width box.
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Old 6th Oct 2018, 8:05 am   #46
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

The DNO are replacing the old separate cables (was a TT system) with ABC (aerial bunched conductors) as part of the conversion from TT to a PME earthing system, they are bunched together to help prevent the dreaded loss of Neutral (breaking of a single wire) condition that is more dangerous with a PME system, as has been mentioned on here before, being as how the N is also the Earthing conductor (TNC) on the DNO supply side, and then becomes TNC-S on the consumer side.

Edit: PME isn't the only TNC-S system, there is also PNB, or protective Neutral Bonding system although this is comparatively rarer and is usually restricted to pole mounted transformer arrangements as far as DNO supplies are concerned .
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Last edited by Red to black; 6th Oct 2018 at 8:15 am.
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Old 6th Oct 2018, 10:06 am   #47
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

I specifically asked the DNO about whether the switch to ABC would mean a change of grounding arrangement. The answer in this case is no, I will (thankfully) retain TT. They're only changing to ABC for maintenance/supportability reasons, the original two-open-wires installation being at least 35 years old.
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Old 6th Oct 2018, 10:58 am   #48
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

If you have a relatively low Earth loop impedance (sub ohm) you will probably be already on a TNC-S system, practically all new public supply systems are now this as it is cheaper for the DNO to roll out.
No new TNS systems are being supplied, even existing ones are to be treated as PME (worst case) due to network repairs in the road so to speak (old lead sheaths breaking down and being repaired by being bridged with the N underground) which results in a sort of hybrid system as there may not be a CNE (common Neutral Earth link) in the actual service head in the property.
Old overhead TT systems are also being converted to PME, look for the Earthing conductor/rod applied to every third pole (or thereabouts) on the overhead systems.

I have had such information from the likes of UK Power Networks and others to this effect as we keep in touch with these bodies on an on going basis.

It may well be that if you happen to be "rural" that they may keep it as TT and are just using the materials they are rolling out elsewhere, obviously I cannot say with absolute certainty, but that has been the jist from what I have gathered so far on dealing with various DNOs over the last few years.
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Last edited by Red to black; 6th Oct 2018 at 11:27 am. Reason: Additional information
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Old 6th Oct 2018, 2:14 pm   #49
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Surely the reason TNC-S is so widely used in spite of such an apparently dire failure mode, is the fact that the multiple redundant connections make it so much less likely to fail that way in the first place?

Which leads us to the interesting question ..... How are the homes of the families of the utility company executives wired up?
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Old 6th Oct 2018, 2:43 pm   #50
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Default Re: Quick-and-dirty testgear: mains voltage monitor.

Quote:
Originally Posted by Red to black View Post
It may well be that if you happen to be "rural" that they may keep it as TT and are just using the materials they are rolling out elsewhere, obviously I cannot say with absolute certainty, but that has been the jist from what I have gathered so far on dealing with various DNOs over the last few years.
Yes, very much 'rural' here. I want to keep my TT setup because it makes things a lot easier regarding not 'importing' potentially RF-noisy earths from the DNO.

[Not sure what a DNO would think about my RF-grounding network: they'd probably ask "What's a radial?"]
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