3-RCD consumer unit 15 ways or more?

[JohnW2]

I'm pretty much decide then, dual rcd it is. ... I guess I now need to decide which one of the many consumer units to go with. I'm after something with a fair amount of wiring room that will allow a nice tidy job. I imagine there will be a lot of personal preference but if there's a no-brainer unit that's great quality I'd like to know about it?

As has been said, if you want to also have an RCBO or three, make sure you get one of those 'high integrity' ones, with three neutral bars and three L busbars.

If you are going to have that 'RCBO or three', one of the most important practical features is to have a reasonable amount of space 'at the top' (above the MCBs etc. The Wylex ones aren't (IMO) too brilliant in that respect, but are usable.

Kind Regards, John

 

[dhutch]

Why is that?

Because the Ze in a TT installation is 'inevitably' (virtually always) so high that the fault current resulting from an L-E short will not be high enough to cause an MCB or fuse to operate.

Continuing a slight digression, but surely the non-rcd cable to the outbuilding is earthed back to the house. And then once in the outbuilding you have a CU and RCD protection from then onwards.


Daniel

 

[THRIPSTER]

Hello Steverog,

You sound as though you pretty much know what you are doing. Just a check though - installation sounds quite large - what is the incoming fuse size and what is your maximum demand? That is, have you got two cookers, an induction hob, three showers, garden pool etc etc.

Not meaning to interfere in any way.


Regards

 

[JohnW2]

Why is that?

Because the Ze in a TT installation is 'inevitably' (virtually always) so high that the fault current resulting from an L-E short will not be high enough to cause an MCB or fuse to operate.

Continuing a slight digression, but surely the non-rcd cable to the outbuilding is earthed back to the house. And then once in the outbuilding you have a CU and RCD protection from then onwards.

Sure, but if its a non-RCD circuit (at the house end) in a TT installation, the the SWA itself would not have satisfactory fault protection, for the reason I gave.

Final circuits in the outbuilding would, as you say, be adequately protected by the local RCD, but that RCD would be oblivious to any fault which arose in the SWA.

Kind Regards, John

 

[dhutch]

Sure, but if its a non-RCD circuit (at the house end) in a TT installation, the the SWA itself would not have satisfactory fault protection, for the reason I gave.

Final circuits in the outbuilding would, as you say, be adequately protected by the local RCD, but that RCD would be oblivious to any fault which arose in the SWA.

Kind Regards, John

But assuming its SWA cable, would the outer not be conected back to the house MET and hence provide a good clean trip?

I can see the argument if the SWA where to terminate as it came into the building, and was then run inside the TT'ed outbuilding in T&E.


Daniel

 

[JohnW2]

But assuming its SWA cable, would the outer not be conected back to the house MET and hence provide a good clean trip?

No - I think you are probably missing the point/problem.

Say someone put a spade partially through the SWA, resulting in a short between L and E (the armour of the SWA). As you say, that armour would be connected to the house's MET (hence CPCs and exposed-conductive parts throughout the house). Given the relatively high impedance of the TT earth (in comparison with an assumed very low impedance of the L-E short in the cable), the potential of the MET, hence all the exposed-conductive parts in the house, would rise to close to L potential (i.e. 230V or whatever).

However, in the absence of an RCD, there would usually be no 'trip'- so those dangerous voltages on exposed-c-ps would persist 'indefinitely' (rather than being disconnected rapidly, as required by the regs). Imagine that the resistance of the TT electrode was about 50Ω (not a bad figure for a domestic electrode) and ignore the resistance of the SWA itself. An L-E short in the SWA would therefore result in a fault current of about 4.6A - not enough to trip even a 6A MCB, let alone one of higher rating. That's why one can't hope to get acceptable fault protection in a TT installation from OPDs (MCBs or fuses).

In contrast, if the SWA were RCD protected, any L-E fault current above 30mA would cause the RCD to operate, clearing the fault.

Is that any clearer?

Kind Regards, John

 

[dhutch]

But assuming its SWA cable, would the outer not be conected back to the house MET and hence provide a good clean trip?

No - I think you are probably missing the point/problem.

Say someone put a spade partially through the SWA, resulting in a short between L and E (the armour of the SWA). As you say, that armour would be connected to the house's MET (hence CPCs and exposed-conductive parts throughout the house). Given the relatively high impedance of the TT earth (in comparison with an assumed very low impedance of the L-E short in the cable), the potential of the MET, hence all the exposed-conductive parts in the house, would rise to close to L potential (i.e. 230V or whatever).

I knew I must be missing something obvious, and as I think your aware (but for the sake of others) I am a mechanical engineering, not an electrical engineer or electrician.

But I think the missing link, is that you are talking about a situation where the houses, aka supply end, is on a TT system. Rather than the situation where the supply is (say) PME going to a outbuilding that is TT.

At which point, I agree and shall now hush up!

Daniel

 

[JohnW2]

I knew I must be missing something obvious, and as I think your aware (but for the sake of others) I am a mechanical engineering, not an electrical engineer or electrician.

... and, just for the record (for the sake of anyone!), and as I think you are also aware, I am not any of those three things

But I think the missing link, is that you are talking about a situation where the houses, aka supply end, is on a TT system. Rather than the situation where the supply is (say) PME going to a outbuilding that is TT. At which point, I agree and shall now hush up!

Ah, I see - now I understand the apparent 'communication problem'! Yes, as I would hope would be apparent if you read your way through all I've said in this thread about TT, I have been talking throughout about a 'TT installation' - i.e. the whole (house) installation having a TT earth, not a separately TT'd outhouse connected to a TN-earthed installation (a possibility that has never actually been mentioned in this particular thread).

Lest any other readers have been confused by all this, let me calrify and re-iterate that if the house has a TT earthing system, then the cable from house to outbuilding must have RCD protection (i.e. at the house end). If the house has a TN earth, then it's usually better when possible) for SWA from house to outhouse not to have RCD protection (at the house end), RCD protection being provided in the outhouse for the local final circuits.

Kind Regards, John

 

[STEVEROG]

Thanks again for all the interesting input.

Regarding details of my installation.....

Here's the detail...

100 Amp Main Fuse

Circuits....

1. Smoke Alarm Circuit
2. Lounge + Study+ Hallway light circuit = Nine lamps
3. Lounge + Study sockets (ring)
4. Shower (10 kW on 10mm cable)
5. Cooker + HOB (on 6mm cable) -well inside diversity
6. Kitchen and Utility room sockets (ring)
7. Boiler
8. Kitchen + Utility room lighting (7 lamps)
9. 1st floor lights (~ 5 lamps)
10. Bathroom lights (1st and ground floor bathroom)
11. Front ground floor rooms and hallway lights (4 lamps)
12. 1st floor sockets (ring)
13. Front ground floor rooms and hallway sockets (ring)

Some of the lighting circuits could probably be paired-up as they're pretty light on load. I'd need to check but I think they're all 1.5mm cabled. Given that I've got many lamps now as LED the loading is ridiculously low.

I'm going to pair the smokeys up with one of the lighting circuits.

There's a second cooker (on spur from kitchen ring) but this is a 13A plug type oven. I do plan to put another 6mm cable in when the opportunity arises. I've not had any overload shut-off occurring on the kitchen ring but given the potential maximum load (washing machine, kettle, tumble dryer etc) I think this might be sensible.

Like many properties, the wiring has been added to over the years. I'd wire it differently if I were starting from scratch but I guess it is what it is.

Existing CU is a Hagar unit, not enough ways to serve the above (some circuits aren't connected yet). I may try to get a larger Hagar so I can re-use the switch, MCB's and RCD but I'm not totally decided on that yet. The Hagar box does seem a bit flimsy.

Thats my lot for now

 

[EFLImpudence]

The Hagar box does seem a bit flimsy.

But the innards are the best.

 

[STEVEROG]

I've heard many folk sing the praises of Hagar, I'm now considering sticking going this route.....perhaps make up a nice sturdy rosewood cabinent for those lovely innards!

Oh BTW forgot to say....

I think my total load is gonna be about 85 Amps based on the old first estimate method of 40% of total breaker values. A bit close but inside.

Other point to note is the 1980's lash up outbuilding power thats taken from the kitchen ring. I'm not particularly happy about that! I've left it connected for now so my garden beers that are sleeping in the shed fridge don't get warm but the plan is to run a new cable back to the CU in the Summer.

 

[JohnW2]

I think my total load is gonna be about 85 Amps based on the old first estimate method of 40% of total breaker values. A bit close but inside.

Maybe, but I very much doubt that, in reality, you'll get anywhere near 85A except possibly occasionally for extremely brief periods.

Kind Regards, John

 

[Adam_151]

I think my total load is gonna be about 85 Amps based on the old first estimate method of 40% of total breaker values. A bit close but inside.

Maybe, but I very much doubt that, in reality, you'll get anywhere near 85A except possibly occasionally for extremely brief periods.

Kind Regards, John

Further to above,

Given an integration period of half an hour then you an perhaps take just 10% of that value unless you have anything strange going on... data farm searching for aliens, cure for cancer or bitcoins, or funny plants in the attic

 

[JohnW2]

Maybe, but I very much doubt that, in reality, you'll get anywhere near 85A except possibly occasionally for extremely brief periods.

Further to above, ... Given an integration period of half an hour then you an perhaps take just 10% of that value unless you have anything strange going on... data farm searching for aliens, cure for cancer or bitcoins, or funny plants in the attic

Quite so - and you'd be very hard-pressed, even with any of those activities, to get half-hour averages even remotely close to 85A. Even when there are transient very high loads, average domestic demands are amazingly small - just as well, otherwise electricity bills would be horrendous!

Kind Regards, John