Outside electrics - circuits

[Tim Watts]

Just bouncing some ideas - as in "what would you do"?

I don't know exactly how my outside electrics will pan out - but I can see some definates:

3-4 double waterproof 13A sockets under the bungalow eaves on each side of the bungalow (that's 1 per side). For misc use and maybe a temporary small shed feed via a centenary for a light and dehumidifier.

Possible long term 20 or 32A feed to a small wooden workshop.

Some random outside lights - 6A

Future pond supply (waterfall, lights) - 6A probable.

The max load of all of that is unlikely to exceed 32A (workshop heater plus powertool plus vacuum). I am not going to be mig welding at the same time as running a shed full of tools and heater!

I bought, speculatively, when I installed the house CU a Double Pole 32A Type B RCBO - with a view to the workshop supply. Double pole allows the possibility to TT the external circuits, but I doubt I'll bother with that.

Idea =====

Rather than wedge more RCBOs in the main CU, I am thinking it might be quite sensible to run this as a distribution circuit to a small 2nd Cu with MCBs for the various final circuits.

I'm well aware the MCBs will not discriminate well with the main RCBO - but that is not a concern - if the "outside" trips, it trips. The MCBs allow suitable cable sizing for each circuit and for partial isolation (partial because the MCBs are single pole) whilst remaining flexible and cost effective.

Any alternative ideas?

Tim

[Bill Wright]

Not many DIY projects take that long.

Bill

[Roger Mills]

Oh, I thought that maybe the shed's 100th birthday was being celebrated.

[Tim Watts]

Actually the old shed looks like it's 100 years old. Getting replaced in a couple of months...

[John Rumm]

That is what I did - that way all outside stuff is easily to isolate from the house supply (and RCDs).

Probably not a concern most of the time, but might be in a workshop with power tools spinning down in the dark. One solution is emergency lights in the workshop.

My cable size was preselected for me (i.e. already there when I moved in). But I replaced the head end MCB with a HRC fuse carrier in the hope that it would discriminate better with the downstream MCBs in the garage / workshop CU. That is a split load TN-S style CU (switch incomer and separate 30mA RCD) - The feed to it is from the main TT CU though, so there is a 100mA RCD at the head end of that.

Hence the workshop lighting circuit is run directly from the 100mA RCD side of the supply and the power from the local 30mA RCD.

[Tim Watts]

Yes indeed - I think at least one unmaintained fitting would be a very good idea in a workshop.

Cool.

Thank you :)

[Tim Watts]

On an aside - do a few overload (no short circuit/fault) trips cause degradation to domestic breakers?

One of my 32A RCBOs was horribly abused when I moved in - running the only working ring in the house with electric heaters everywhere, before I got a full set of proper ring circuits installed.

It overloaded and tripped a few times.

Now I'm finally finishing up and tidying up, I'm wondering if I should regard it as suspect. It checks out fine OK with a Megger on the RCD tests (ie trips correctly at the right levels in the right time).

The reason I wonder is based on an old story my father told me concerning the big breakers at substations (he worked for the LEB). Apparently after a fault trip (breaking short circuit currents) the arc quench oil had to be filtered to remove carbon residue.

Obviously a very different device to an MCB/RCBO - but it did make me wonder if the latter could sustain contact damage or even overheating from the thermal trip element from too many operations.

[meow2222]

Only in the imagination does a few become too many, and a device designed and certified to do a job become hopelessly incapable of doing it. It seems extremely unlikely you'd have an mcb that has its own oil bath.

NT

[Tim Watts]

Never said that -

But I do wonder what their max (full load x say 1.5) operating cycle are and what number of cycles clearing a fault.

I suspect (would hope) very large.

But the fault clearing is a perfectly good point. Under a fault, you are expecting the contacts to break a current of 100's amps (PSCC at my incomer 1.2kA so a dead short near to RCBO would realise a very large current).

That's going to burn and pit any small contact pair.

The overheating point is not totally insane either - I've seen a recommendation that higher rated RCBOs are not packed side by side without leaving a blank every 2-3 due to heat build up. This assumes the neighbouring devices are heavily loaded all the time. Don't have a quotable source but it was mentioned on the IET forums a few years back.

It's not intuitive but then neither is the fact that a double 13A socket cannot be loaded to 26A :)

[John Rumm]

IIUC, repeated tripping at fault current magnitude may over time wear the contacts (but I expect you would need lots of trips at close the the breaking capacity).

For normal thermal trips the effect should be noticeably less. The arc quench mechanism is usually just a convoluted air path - so no oil etc to worry about.

If there is no obvious thermal damage to the case etc, then I would not be worried - there are not really any particularly thermally sensitive components in them that will degrade with heating.

[ARW]

I have one idea. Use a non RCBO supply to the second CU (SWA supply or similar) and use all RCBOs or a split RCD unit with some RCBOs and some MCBs on a shared RCD on the second CU. Distribution boards only need a 5s disconnection time from the main MCB............

[Tim Watts]

Thanks Adam.

That is certainly viable as the 2nd CU would be "round the corner" from the main CU so arranging a cable run that would be exempt from RCD protection requirements is possible (surface run through the attic space).

[Tim Watts]

I really must take an old MCD to bits to see that :)

What do they face the contacts with that is burn resistant?

This is an RCBO so I was wondering about the electronics.

[Andy Wade]

BS EN 61009 requires a s/c rating - making & breaking - of at least 3 kA[*], so no problem there.

[*] Except for an RCBO built into, or designed to be used only with, a socket-outlet, which can be 1.5 kA.

[Tim Watts]

Thanks Andy - I'd love to know what metal they use on the contact faces.

[John Rumm]

Also worth noting that most MCBs and RCBOs are rated to 6kA nreaking capacity these days (the "plug in" BS3036 wylex fuse replacement type being one notable exception)