Another electricity supply question

On Mon, 26 Jun 2017 08:48:26 +0100, Graeme wrote:

Switching the shop to supply to a no cost option (remove it or no standing charge tarrif with zero use) gives you £245/year, less cost of 1500 units on the house supply, perhaps £100/year. guesstimate £400 to get the work done, 4 years...

he

Snag with that is if you turn off the house supply for any reason the shop goes off as well. Split the tails, fit the required switch fuse into the sub-main to the shop CU and the shop can be on with the house off or vice versa.
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Dave.
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Even better! Yes, cutting the power to the shop always sends the alarm into a tizzy, so best avoided if possible. It is an ex Post Office, so fully alarmed even though no longer in service.
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On 26/06/2017 15:44, Graeme wrote:

Power cuts setting off alarms are often cured by replacing the battery in the alarm control box.
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The battery is fine. The alarm does not go off, but the control box emits constant warning beeps, and requires resetting.
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On 26/06/2017 12:39, Dave Liquorice wrote:

It's not much of a snag is it?
How often does the average householder have to turn the house supply off?
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On 26/06/2017 08:48, Graeme wrote:

I know it's not much load, 1500 unit pa.
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On Mon, 26 Jun 2017 19:01:26 +0100, ARW wrote:

shop

cable

Poor assumption, it could be a 16 kW load for 30 mins every other day:
16 * 0.5 * (365 / 2) = 1460 kWhr.
The 24/7 load is a mere 170 W (0.17 * 24 * 365 = 1489 kWHr).
In reality the peak load will be somewhere between the two, maybe 4 kW for a kettle plus a 1 kW of other load but remember *everything* is going through that single MCB in the house CU, so you loose diversity. Overload the house CU MCB and everything goes off in the shop. Overload an MCB in the shop CU fed with a sub-main only that MCB goes off, perhaps the lights stay on...
Not mention that having a seperate supply available when the other is off for some reason is handy.
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On Monday, 26 June 2017 21:23:59 UTC+1, Dave Liquorice wrote:

It depends. If you have say a 32A & 6A shop circuit fed from a >38A MCB then you're unlikely to trip both circuits if one trips.
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On 26/06/2017 22:28, snipped-for-privacy@gmail.com wrote:

Not if it's a fault current:-)
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On Monday, 26 June 2017 23:22:59 UTC+1, ARW wrote:

The graphs at http://wiki.diyfaq.org.uk/index.php/MCB show that discrimination is normally obtained with fault currents.
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On 27/06/2017 09:34, snipped-for-privacy@gmail.com wrote:

not all fault currents have looked at the graph though ;-)
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On Tuesday, 27 June 2017 14:05:49 UTC+1, John Rumm wrote:

In most cases discrimination will be maintained. The fact that ocasionally it isn't is rather trivial, and hardly worth a protracted discussion. The op knows the options.
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On 27/06/2017 19:51, snipped-for-privacy@gmail.com wrote:

How on earth can you say "in most cases discrimination will be maintained" from those graphs and keep a straight face?
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On 27/06/2017 19:51, snipped-for-privacy@gmail.com wrote:

In many cases of *overload*, discrimination will be maintained - at least with a single downstream MCB one rating step or more lower than upstream.
With multiple MCBs downstream (say a CU on a submain) its probably still likely - but somewhat less so.
However with a 500A fault current why would you expect the magnetic response of the lower trip current device to be necessarily faster than that of the higher rating device?
(its for this reason one often elects to use a HRC fuse for the head end of a submain)
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On Thursday, 29 June 2017 03:38:22 UTC+1, John Rumm wrote:

Of the cu trips & fusings I've seen, not many have been such high current. The op can complicate the rewire, but I'm not covinced there will be much gain.
NT
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On 29/06/2017 03:45, snipped-for-privacy@gmail.com wrote:

Who you kidding? 500A of PSSC only requires a Ze of around half an ohm or less, common on many installations.
However the issue still stands at lower currents - say 180A then... still enough to trip a B32 MCB in a head end.

How would be using a HRC fuse in a carrier in the head end CU "complicating" it? Its the same design, just a different choice of protective device.

There won't - hopefully there won't be many cases where fault current protection needs to operate on the downstream installation in the first place. If it does, and you trip multiple MCBs, then that may be acceptable in the circumstances for an infrequent situation.
However that does not mean you can get away with glib "...show that discrimination is normally obtained with fault currents" statements! ;-)
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On Thursday, 29 June 2017 12:14:43 UTC+1, John Rumm wrote:

the point is that most trips aren't due to a zero ohm short

I didn't say it was. Running multiple cables to the shop & removing the CU would be.

so despite arguing you agree anyway

40A MCB takes more i squared t than 32A & 5/6A MCBs to trip. With a 5/6A circuit problem you'll nearly always get discrimination. With the 32A circuit you often will.
As I said you can complicate the job by losing the shop CU & adding more cables, but the real life gain will be minimal.
NT
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On 29/06/2017 12:26, snipped-for-privacy@gmail.com wrote:

If it were my setup, I would run a submain from house CU to shop CU. Leave the tails from that meter parked in a henley block and disconnect the service. I would put a suitably sized HRC fuse in a fuse carrier in the house CU for the submain. Minimal change and new materials. The OP could then have the service to the shop terminated and let the supplier do what they want with the meter and tails etc.
As a DIY job its the cost of the submain cable, and probably no more than a couple of hours of work - would likely pay for itself in the first year.

I am disagreeing with your statement about discrimination, since it was basically wrong for the specific case of handling fault currents.
That is a separate issue from the solution that I propose to Graeme, since in this circumstance I suspect he could cope with a lack of discrimination given the circumstance he describes, and the likelihood of needing to clear a fault in the first place is relatively low. However since some mitigation can be designed in at no extra effort or expense it would seem daft not to.

True for overloads, but not faults.

No point since there are easier ways, and for that matter *in this circumstance* Graeme may not care anyway.
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On Thursday, 29 June 2017 18:40:53 UTC+1, John Rumm wrote:

So in short you do agree that it will generally discriminate on overcurrents, but not on dead shorts of zero ohms. Glad we cleared that up.
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On 29/06/2017 18:40, John Rumm wrote:

Just to add to that suggested set up - and I believe that it may be important.
The old shop CU may well be include some form of main equipotential bonding that is specific to the existing shop CU which would be lost if the supplier physically removes their supply. You would have to run a 10mm earth along with the submain cable to incorporate any bonding back to the house CU.
As for the fault current.
I'll give an example of a 40A MCB feeding a CU that has a 32A and a 6A MCB. In this case I am going for a non RCD protected 40A MCB supplying a RCD CU. I assume that this could be a realistic option for the OP although I know very little about his setup.
The max ELI of a 40A MCB is 1.15 ohms and a current of 200A is needed to trip the 40A MCB in a fault condition.
So the maximum Zs at the shop CU is 1.15ohms
Now a 32A MCB only needs 160A to trip in a fault condition and so can have a maximum ELI of 1.44ohms.
Now what happens[1] if the shop CU has a Zs of 1.00ohms and a fault occurs somewhere on the 32A circuit at a point where the Zs is less than 1.15ohms?
[1]Extra marks for working out the maximum 2.5mm ring circuit size that will keep the maximum Zs under 1.15 ohms.
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