TN Distribution circuit to an outhouse

[Matty7814]

Evening gents,

I'm just looking for some guidance/clarification on a supply to a summer house I installed. I had to put in a small CU (the existing one had no spare ways left) with a 63A 30mA RCD main switch and a 45A B type feeding the distribution circuit. 6mm2 2 core SWA around a 25m run to the summer house that fed a garage CU with 40A 30mA RCD main switch.

The small CU I put in the house had a Ze of 0.2 ohms and the Distribution circuit had R1 +R2 of 0.33 ohms. The Zs of the circuit/Ze of the garage CU was 0.79 ohms.

Now looking at table B6 in the OSG shows that max measured Zs for 45A b type is 0.82 but I wasnt sure if that includes distribution circuits. Reg 411.3.2.3 states that for a distribution circuit a disconnection time not exceeding 5 seconds is permitted and 411.4.5 says: Zs x Ia < U0

Am I right in thinking that 0.79 x 225A = 177.8 so that it complies?

Apologies in advance for the wall of text and as you might of gathered I'm fairly new to this!

Thanks for your time

Matt

 

[ImpededLoop]

Hi,

If your Zdb at the shed is 0.79 ohms and the max permitted in the OSG is 0.82 ohms, then your circuit meets the required disconnection times, especially as it has an upfront RCD.

The use of an RCD at the supply end of the submain means that you didn't also need one in the shed as both will likely operate at the same time.

You are also correct that a distribution circuit can have a 5 second disconnection time.

Have you calculated the volt drop for 6mm cable over that distance as it sounds quite close to the limit if you're designing for a load approaching 45A (based on your choice of protective device)

Finally, your calculated Zs would be 0.53 ohms, so you may want to look at why your measured Zs is 0.79 ohms.

 

[Matty7814]

HI,

Thanks for the response. Yea during the RCD test it tripped at both ends a couple of times so that one to note for the future I guess.

I calculated the Voltage drop on a load of 26 amps as there is going to be a 20A socket radial and 6 Amp lighting circuit in the shed which came out to 5.2V/2.2% . Should I have calculated that using the 45A Breaker then I guess?

Yea I did wonder why there was a .26 ohm difference in the two Zs values, I will investigate that when I go back.

Another question I have is should the RCD on the supply side that acts as a main switch be DP? I couldnt find anything in the regs that said a Main switch had to be DP but I'm not 100%

Thanks again

Matt

 

[Taylortwocities]

I suggest you take the RCD out of the small board you fitted in at the house end and install a standard DP main switch.
That will sort out the twin RCD issue.
DP isolation is always best, especially on external circuits where isolating neutral as well as line can be important.

I guess you measured your Zs using the EFLI measurement on your multi-tester with a no-trip feature (D-Lock) etc?
I don't know the make of your tester, but some have a difference between measured Zs as compared with the Zdb+(R1+R2) due to the sneaky way the D-lock technology creeps up on RCDs.

That may explain the difference.

 

[Dave OCD]

Yes it definitely should - as you say it's a main isolator which must disconnect all current carrying conductors - meaning both live and neutral.
But relax, all RCD's are DP - obviously this doesn't apply to many RCBOs though.

 

[Matty7814]

Cheers guys,

This was my first bit of private work and I panicked at the last minute and had both RCDs as I wanted the SWA RCD protected but then didnt think that the ones in the shed would be protected by that same one! Also got lost with discrimination and wasnt sure if I could have the same size CB in a row so thats how I ended up with 63A Rcd>45A CB>40A RCD> final circuits in shed. Clearly more revision needed!

Also I have a MFT 1730

Cheers

Matt

 

[Dave OCD]

We all had to learn and gain experience Matt and you never stop really, never be afraid to ask if you're unsure of anything though mate.

 

[Andy78]

Cheers guys,

This was my first bit of private work and I panicked at the last minute and had both RCDs as I wanted the SWA RCD protected but then didnt think that the ones in the shed would be protected by that same one! Also got lost with discrimination and wasnt sure if I could have the same size CB in a row so thats how I ended up with 63A Rcd>45A CB>40A RCD> final circuits in shed. Clearly more revision needed!

Also I have a MFT 1730

Cheers

Matt

I would agree that more revision is needed before you work again unsupervised for a private customer. I would also say that more experience under supervision would enhance your understanding of your job. There are massive gaping holes in your knowledge based on your statements in this thread that just shouldn't be there for a professional undertaking paid work.

 

[Matty7814]

Looking back into OSG 3.6.1, it doesnt seem that my distribution circuit required an RCD as it is SWA. All points to note for the next job I guess.

Cheers Dave

Matt

 

[Matty7814]

I would agree that more revision is needed before you work again unsupervised for a private customer. I would also say that more experience under supervision would enhance your understanding of your job. There are massive gaping holes in your knowledge based on your statements in this thread that just shouldn't be there for a professional undertaking paid work.

Yea that would be ideal to be honest. The firm I started working for said I would work alongside an experienced spark but that stopped after about a week and all they seem to do now is get someone to check my work/snag it and sign it off. Tying to find someone willing to take on a 28 year old mate has been difficult unfortunately :snore:

 

[davesparks]

Cheers guys,

Also got lost with discrimination and wasnt sure if I could have the same size CB in a row so thats how I ended up with 63A Rcd>45A CB>40A RCD> final circuits in shed. Clearly more revision needed!

Also I have a MFT 1730

Cheers

Matt

The 45A mcb won't discriminate fully with the 20A or possibly even the 6A mcb
The RCDs won't discriminate with each other at all, which ever one has been calibrated to trip faster will do so.
The problem you do have is that you have a 40A RCD supplied by a 45A mcb so the RCD could be overloaded without the OCPD operating

 

[Leesparkykent]

Where about's in London are you?

 

[Darkwood]

You cant usually achieve discrimination in domestic and only partial at best ... this applies to most situes where you feed a submain off a mcb, having said that it doesnt warrant any design changes because nuisance tripping of the wrong ocpd isn't really a big concern in domestic.

 

[Matty7814]

Cheers Dave

If I changed the 45A to a 40A would that discriminate with the 40A RCD?

 

[Matty7814]

Where about's in London are you?

I'm in Islington North London