Another electricity supply question

[tabbypurr]

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

[John Rumm]

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! ;-)

[tabbypurr]

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

[John Rumm]

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.

[tabbypurr]

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.

NT

[ARW]

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.

[tabbypurr]

It does strike me as missing the point once again. Most MCB trips are caused by overcurrent rather than a dead zero ohm short.

NT

[ARW]

Care to state the reference for "Most MCB trips are caused by overcurrent rather than a dead zero ohm short."

>

[tabbypurr]

experience.

NT

[Graeme]

In message , John Rumm writes

I'm inclined to follow John's advice, keeping in mind Adam's earthing comment. Thanks all. I confess that at least some of the discussion is above my head, and whilst I can do some of the job, I'll leave the actual connections and disconnections to a pro.

[John Rumm]

That also seems suspect to me.

One will normally select and design circuits to be adequate for their expected use, so overloads should be the exception rather than the norm. While a design can be fault tolerant, its much harder to design the possibility one will occur at all out of it.

I can only think of one example where I have seen a MCB do a thermal trip, but loads on the magnetic response. (for example, filament lamp failures, faulty appliance, faulty lamp, good old fashioned cable damage (watching someone chisel through a live cable!), drilling into one myself and so on.

[tabbypurr]

That can be true, depending how you define overload. Eg it's normal for rings to supply much more than 32A, OTOH circuits shouldn't regularly trip.

impossible

Excess current that is not a dead zero ohm short can trip them in either thermal or magnetic mode. A failament lamp failure or faulty motor are good examples of this. Drilling into a 32A cable is of course more a dead short.

NT

[John Rumm]

If you can decide on the cable required, and lay that in, it will cut down the amount of work required.

Regarding the main equipotential bonds, there are some things worth checking. Firstly find out what earthing system[1] is used in the shop and the house. For example if it's TN-C-S (PME) then you will need to ensure that the house's main equipotential zone is extended[2] to the new CU. Also does the shop have other independent incoming services? (e.g. water, gas etc). If it does, chances are those were previously included in the main bonding to the shop's earthing system, but you will need to check.

[1] [2] Although in this case discussing feeds to outbuildings, much of what is described could apply here:

[Graeme]

In message , John Rumm writes

This, I confess, is where I get lost. Several times, over a good few years, I have read the Wiki article and looked at the incoming supply, but am still none the wiser. I'll need to take good photos of both CUs and post them here, in the hope that someone will be able to see exactly which system is in use. Alternatively, if I find a proper electrician to handle the actual connections, he will know.

[Dave Plowman (News)]

Means you must have experienced lots of bad design, then?

[John Rumm]

Yup photos would be good. Especially if they are of installs that are different in look from the ones we already have - then we can pinch em to update the article with ;-)

[tabbypurr]

no.

NT

[Dave Plowman (News)]

How else do you explain regular over current trips, then?

[charles]

Halogen lamps failing is very common cause. Mind you, they often take out a domestic dimmer, too.

[tabbypurr]

Where do you get regular overcurrent trips from? If you keep getting them, something is wrong. Sometimes the things you say are a bit odd.

NT