New circuit design question

[RyanAspire]

We haven't established 70mm is correct as we do not have enough information. So the supply to the kiosks from kiosk 2 will be rcd protected?

I have based it on 100A. And yes, that's what I am intending, albeit I may change the circuits to RCBOs at each kiosk instead of a main RCD. Except for the consumer unit in the mobile home

 

[westward10]

Bad idea giving the other kiosk supplies rcd protection if they do not require it. Do the external lights require rcd protection?

 

[loz2754]

Call me out on this if y'all feel I'm too harsh...
But surely this is basic bread and butter circuit design 101.
Which any trained electrician should be able to do, it's part of the job.
I think we're seeing far too much of this "design by internet forum" approach.

 

[RyanAspire]

Call me out on this if y'all feel I'm too harsh...
But surely this is basic bread and butter circuit design 101.
Which any trained electrician should be able to do, it's part of the job.
I think we're seeing far too much of this "design by internet forum" approach.

I wouldn't call you out for it, you would be right had I asked this without having a clue but I haven't stated that anything I'm doing is drastically any different to what others have said. I was just asking a question as I felt maybe the wording in the regulations could portray a different scenario.

The RCD query was more to ensure I wasn't missing anything obvious as I am trying not to overspend on the job.

And obviously the earthing question was more as I've never worked with mobile homes/caravans and its good to get some more background from those that may know more about it. Albeit it seems that what I 'knew' to be correct, is.

 

[RyanAspire]

Bad idea giving the other kiosk supplies rcd protection if they do not require it. Do the external lights require rcd protection?

Probably not. The RCD idea was mainly so if there was more circuits in the future then it wouldn't entail more RCBOs (at higher cost) or if a different spark comes in and tries rewriting the wheel to get more money from the job too. I think seperate RCBOs is the way forward though to prevent the 'nuisance trip' scenario

 

[RyanAspire]

Also, I'm sure i know what the answer is to this one....the supply is TNCS for reference, but I won't need an earth rod for caravan will I? I'm not looking to create a TT system or PME unless it is beneficial/regulation.

ESQCR prohibits the use of PME for pikey vans. Start banging those rods in!

I've got the mallet ready ?

 

[loz2754]

I wouldn't call you out for it, you would be right had I asked this without having a clue but I haven't stated that anything I'm doing is drastically any different to what others have said. I was just asking a question as I felt maybe the wording in the regulations could portray a different scenario.

The RCD query was more to ensure I wasn't missing anything obvious as I am trying not to overspend on the job.

And obviously the earthing question was more as I've never worked with mobile homes/caravans and its good to get some more background from those that may know more about it. Albeit it seems that what I 'knew' to be correct, is.

Fair comment.
Perhaps if we had been presented with a more or less complete design, with a request for alternative thoughts and ideas, I would not have got the wrong idea. My apologies.

 

[RyanAspire]

Fair comment.
Perhaps if we had been presented with a more or less complete design, with a request for alternative thoughts and ideas, I would not have got the wrong idea. My apologies.

I get what you mean though, when I did my 18th edition I'd say half the class were struggling with it, and i really couldnt grasp it, I wonder how they got through college and then couldn't handle the easiest hurdle of the lot, that is what worries me.

I like the forums for being able to answer these queries though, sometimes for me, like this, it's just curiosity that you're not being a complete idiot about things ?

 

[Spoon]

I have based it on 100A. And yes, that's what I am intending, albeit I may change the circuits to RCBOs at each kiosk instead of a main RCD. Except for the consumer unit in the mobile home

Kiosk 1 incoming from UKPN with meter and fused isolator as you say

You are fitting a fused isolator, so you will not have 100A

 

[pc1966]

What is the load at each of the kiosks? What you really want to avoid is any selectivity issues where a major fault on one takes out the lot.

At the origin you should have your own switched-fuse so you are in control of the current limit (or badger the DNO to agree their fuse is OK for protection, I know which is easier...) and check that the sub-main is going to disconnect in 5s or less if there is a fault. Probably with 70mm and TN-C-S supply that is going to be OK, but get the Ze at the incoming point and check the fuse requirements.

If none of your kiosks has an expected load above 60A then you would be best to chain the supply from the incoming 100A (fused) line to each one and have a fused cut-out (or another fused-switch) at 60A to feed the local CU, that way even a major fault will not take out the incoming DNO & switched-fuse's fuses.

As above, if you are feeding caravans they are not permitted to be from TN-C-S so at the last leg you should be making it a TT supply with local earth rod(s) to be OK, and RCD protection before the feed to the caravans. Just search for "IET caravan power" and you should find an article on that top of the list.

 

[RyanAspire]

You are fitting a fused isolator, so you will not have 100A

Based the calcs on 100A. Really can't help but feel youre here to call me out rather than help?

 

[Spoon]

Based the calcs on 100A. Really can't help but feel youre here to call me out rather than help?

I think you have misread my posts mate. I have given you good info.
You say the main fuse is 100A and you are fitting a fused isolator, therefore the fused isolator can't be 100A as you would not have any discrimination mate. It has to be lower than 100A.
A few people have asked for more info on what power the kiosks are designed at, and rightly so. The more info we have the more we can help.

 

[pc1966]

You say the main fuse is 100A and you are fitting a fused isolator, therefore the fused isolator can't be 100A as you would not have any discrimination mate. It has to be lower than 100A.

There is no requirement for the fused isolator to protect the DNO side, that have their own fuse. It is so you can justify protection of the sub-main without relying on the DNO's permission.

True it won't provide selectivity at the DNO if you use the same value (assuming it meets the sub-main disconnection times) but if you are likely to blow the DNO fuse without a major sub-main fault (pick-axe through it, etc) you have under-specified the supply in the first place!

That is why I suggested a 60A fused-switch at each take-off point, as that should provide selectivity with the incoming fuses of both DNO and a customer.

 

[Spoon]

There is no requirement for the fused isolator to protect the DNO side, that have their own fuse. It is so you can justify protection of the sub-main without relying on the DNO's permission.

True it won't provide selectivity at the DNO if you use the same value (assuming it meets the sub-main disconnection times) but if you are likely to blow the DNO fuse without a major sub-main fault (pick-axe through it, etc) you have under-specified the supply in the first place!

That is why I suggested a 60A fused-switch at each take-off point, as that should provide selectivity with the incoming fuses of both DNO and a customer.

Ok, so in past posts, if someone was to extend their tails from the meter to the CU, and it goes over 3 meters they have been told that they need to install a fused isolator. Why doesn't this apply here then? The principle is the same.
Or have I misread something...

 

[David Prosser]

Ok, so in past posts, if someone was to extend their tails from the meter to the CU, and it goes over 3 meters they have been told that they need to install a fused isolator. Why doesn't this apply here then? The principle is the same.
Or have I misread something...

Yes you have missed the bit that says "There is no requirement for the fused isolator to protect the DNO side". Not there is no need for a fused isolator. The point being they can have 100A fuses the same as the DNO.

 

[pc1966]

As above, my comment was on the fuse size aspect.

 

[pc1966]

A quick look at 70mm has that run coming in at around 0.63 * 122 = 0.077 ohms R1+R2 if 3-core, and a 100A fuse to blow in 5s or less is around 0.3 ohms Zs max, so you need to check the supply Ze is no more than about 0.2 ohms otherwise you are going to need to do something else (e.g. 300mA delay RCD) to meet disconnection limits.

While 2-core is cheaper by a bit, the armour is around 2 ohm/km so the run would have R1+R2 of around 0.277 ohms leaving practically nothing for supply Ze & 100A fuse protection.

 

[Spoon]

Yes you have missed the bit that says "There is no requirement for the fused isolator to protect the DNO side". Not there is no need for a fused isolator. The point being they can have 100A fuses the same as the DNO.

Now I'm confused.
So you are saying that a fused isolator is required, due to the distance from the meter to the first CU.
What's the point in putting in a 100A fuse. We have always recommended a fuse lower than the main fuse size?

 

[David Prosser]

kiosk one can have 100A fuse switch giving you better flexability with your selectivity futher down the chain.

 

[Spoon]

kiosk one can have 100A fuse switch giving you better flexability with your selectivity futher down the chain.

But if there is a fault then either the main fuse or the isolator fuse could blow..