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...
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.
Poor assumption, it could be a 16 kW load for 30 mins every other
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.
On 27/06/2017 19:51, email@example.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
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)
On 29/06/2017 03:45, firstname.lastname@example.org 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
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! ;-)
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.
On 29/06/2017 12:26, email@example.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
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.
Just to add to that suggested set up - and I believe that it may be
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 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
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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