new CU and garage wiring questions

[ban-all-sheds]

MEM, Seimens, Merlin-Gerin/Schneider, .... ?

 

[sherlockholmes]

Is contactum a good brand?

Not considered the best. Cheap though and a 25 year guarantee on their accessories if they manage to exist for that time. They went into administration a couple of years ago but were bought by a Saudi firm, I think. Wouldn't be as confident that their CU stuff would be as easily available as MK, Hager, Wylex for example.

Thank you, that is very useful information. I will request a brand that is still solvent, after all if it goes wrong I shall need to... contactum.

 

[1john]

MEM, Seimens, Merlin-Gerin/Schneider, .... ?

I recall a recent post where Hager were quite popular, i have fitted a few of the new boards recently and they are quite nice to work with.

 

[sherlockholmes]

The first chap knocked a hundred off his price, and has provided by far the best written quote with clear diagrams and a schedule so I'm going with him. Take note sparkies, customers like cogent written quotes and sometimes even accept slightly higher charges if it seems the man knows his business better than the competition.

I have one question, which may be a silly one so I'll ask it here rather than of the electrician -
What is the reason for the large and moderately expensive bit of kit known as a switch fuse? The armoured cable terminates into it, and tails feed it from the henley block. Specifically, what is the advantage over the upstream isolator (one is being put in before the Henley block), which has no fuse but as I understand it does the same job of disconnecting the current in the event of disaster? I was under the impression fuses could now be replaced by handy little devices without any replaceable parts.

 

[JohnW2]

What is the reason for the large and moderately expensive bit of kit known as a switch fuse? The armoured cable terminates into it, and tails feed it from the henley block. Specifically, what is the advantage over the upstream isolator (one is being put in before the Henley block), which has no fuse but as I understand it does the same job of disconnecting the current in the event of disaster? I was under the impression fuses could now be replaced by handy little devices without any replaceable parts.

With the caveat that I am not an electrician .....

As with all overcurrent protection devices, the purpose of the switch fuse (the fuse part of it) is to protect the cable (in this case the armoured one) from damage due to excesive currents. Assuming what you describe as the upstream isolator has a protection device (even though not a fuse) within it (an 'isolator' could just be a manually-operated switch, with no automatic disconnection ability), if it is before the Henley block it will have to be rated to carry the entire current of your electrical installation - and therefore might well not disconnect until current through your armoured cable was sufficient to damage it. The need is therefore for the armoured cable to be protected by a device (fuse or other over current device) which is matched to the current-carrying capacity of the armoured cable. The issue is therefore not whether protection is afforded by a fuse or something else but, rather, what the rating of that device is.

Hope that helps.
Kind Regards,
John

 

[echoes]

... and the DNO's fuse is usually allowed to protect no more than 2-3m of meter tails. If there was no switch-fuse for the SWA, then that entire length of SWA would have no overload or s/c protection other than the DNO's fuse, which would be bad on a number of levels.

 

[ricicle]

Also the switch-fuse will probably be metal-clad so that earth continuity is easily maintained on to the armouring of the SWA. The upstream isolator is possibly not metal-clad.

 

[ban-all-sheds]

... and the DNO's fuse is usually allowed to protect no more than 2-3m of meter tails. If there was no switch-fuse for the SWA, then that entire length of SWA would have no overload or s/c protection other than the DNO's fuse, which would be bad on a number of levels.

Including financially, because it would mean that the SWA would have to be sized to be OK on a fuse the size of the DNO one.

 

[ban-all-sheds]

I was under the impression fuses could now be replaced by handy little devices without any replaceable parts.

The reason that a fuse is preferred is to give better discrimination, i.e. that a fault in a final circuit in the garage will just trip the MCB in the garage CU, not take out the upstream device. If the upstream one was an MCB there would be little or no guarantee of that.

 

[echoes]

... and the DNO's fuse is usually allowed to protect no more than 2-3m of meter tails. If there was no switch-fuse for the SWA, then that entire length of SWA would have no overload or s/c protection other than the DNO's fuse, which would be bad on a number of levels.

Including financially, because it would mean that the SWA would have to be sized to be OK on a fuse the size of the DNO one.

That was indeed one of the higher levels

 

[JohnW2]

... and the DNO's fuse is usually allowed to protect no more than 2-3m of meter tails.

The OP seemed to be saying that there was an upstream 'isolator' which incorporated overcurrent protection, prior to the splitting of the tails. If that's true (sounds slightly odd to me), then that device (not just the DNO's fuse) would be protecting the split tails. However, as I said, that would obviously mean that the device would have to be rated for the entire installation, far too high to protect the SWA (unless ludicrously large/expensive SWA were used!).

Kind Regards, John

 

[sherlockholmes]

I don't know if the planned isolator provides overcurrent protection or not, it's just labelled 100A double pole isolator on the diagram. My intended question would perhaps have been more clearly put as why a switch fuse for overcurrent protection instead of a cheaper, smaller modern circuit breaker. Either could be rated at a level to protect the cable downstream. I think BAS's answer covers it with discrimination.

Thank you chaps, the work is all scheduled and this has been an interesting brief education. Perhaps I shall do a part P course myself and hit the road with my trusty neon screwdriver

 

[echoes]

Perhaps I shall do a part P course myself and hit the road with my trusty neon screwdriver icon_wink.gif

Remember to calibrate it properly....

 

[JohnW2]

I don't know if the planned isolator provides overcurrent protection or not, it's just labelled 100A double pole isolator on the diagram.

Ah, then maybe it is just as isolator then (a manually-operated 'switch' in common parlance, although unlike switches, an isolator is not intended to be used to switch off a circuit 'under load'). It could well be that the electrician has specified an isolator there to faciliate his subsequent work (and that of other electricians in the future). What made me think that you knew it afforded overcurrent protection was that you wrote:

...the upstream isolator (one is being put in before the Henley block), which has no fuse but as I understand it does the same job of disconnecting the current in the event of disaster

My intended question would perhaps have been more clearly put as why a switch fuse for overcurrent protection instead of a cheaper, smaller modern circuit breaker. Either could be rated at a level to protect the cable downstream. I think BAS's answer covers it with discrimination.

Indeed. I thought you were primarily asking why the switch fuse (or some alternative) was there at all (given the presence of the upstream 'isolator'), with the fuse vs. MCB question as a 'secondary' one.

I hope the work goes well.

Kind Regards, John.

 

[JohnW2]

Removed (got posted twice, somehow - my fingers, I guess!! Apologies