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Topic Title: Quick Design Check Please
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Created On: 22 January 2013 07:06 PM
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 22 January 2013 07:06 PM
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andyedtec

Posts: 279
Joined: 28 July 2006

Please could you give my design for some caravan pitches the once over.

Sub Main

60A Bs88 sw fuse supplying a 4 way 100A Main Switch, Metal Clad Board with 3x32a Amp mcbs. Cable size 10mm 3core swa. Cable Clipped direct in loft.

Final Circuits

Each 32 amp circuit supplies a caravan hookup Unit with 2 x 16a sockets with there own rcbo's. Each hookup will be on TT with the armor cut short and cable insulated and glanded with a stuffing gland. Cable is buried.

Circuit 1 lenght 70 M 10mm 2 core swa 32 A mcb
Circuit 2 + 3 35M on 6mm swa. 32 A mcb

Any comments or suggestions appreciated.

Andy
 22 January 2013 08:32 PM
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alanblaby

Posts: 405
Joined: 09 March 2012

You'll need 2 pole RCBOs.
708.553.1.13
 22 January 2013 08:48 PM
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andyedtec

Posts: 279
Joined: 28 July 2006

Originally posted by: alanblaby

You'll need 2 pole RCBOs.

708.553.1.13


Thanks I've covered that.

You will note the sub main is 63A, I have taken diversity in to account and have settled for 10A per pitch, although 2 outlets are supplied by 1 x 32a Mcb. Full load current could be 96A but I dont think it will go over 40 really.

Does that sound about right?
Andy
 22 January 2013 08:49 PM
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Legh

Posts: 3538
Joined: 17 December 2004

A few questions

1/ Where's the 80A BS 88 fuse being fed from?
2/ I assume from this set up its single phase?
3/ What is the max Demand available at the origin and what is being consumed at the point where it supplies the other installation/s ?

Legh

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 22 January 2013 09:17 PM
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AJJewsbury

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You won't get much discrimination between the 16A RCBOs and 32A MCBs on faults (L-N on the TT section and both L-N & L-PE in the TN section) - not sure if that's acceptable or not.

What's the layout like? Would running a single main direct from the BS 88, looping to each hookup, work? (possibly larger cable, but fewer of them and avoiding the cost of a DB, and giving better resilience from a fault in one pitch affecting its neighbour and maybe better use of diversity. Downside would be that all pitches would go off if the main was damaged - but presumably that's a lot less likely that a fault within a pitch. Just a thought.

- Andy.
 22 January 2013 09:26 PM
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andyedtec

Posts: 279
Joined: 28 July 2006

Originally posted by: Legh

A few questions



1/ Where's the 80A BS 88 fuse being fed from?

2/ I assume from this set up its single phase?

3/ What is the max Demand available at the origin and what is being consumed at the point where it supplies the other installation/s ?



Legh


1/ It's a 60A Bs 88
2/ yes
3/ The switch fuse is fed from Henly blocks Next to main Bungalow service head and CU. Demand is very low in the bungalow. A 200A supply has been requested but we are not holding our breath. Hence keeping to 63A.
 22 January 2013 09:32 PM
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cmatheson

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Joined: 16 January 2003

I think its worth considering the situation where the happy camper is merrily resetting the rcbo and those responsible for the site are unaware.

I prefer a single rcd serving the entire site behind a lock (or screwed panel as well as rcbos for individual final circuits) so that, in the event of a fault, the responsible person will be made aware.

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Chris Matheson MInstMC
 22 January 2013 09:35 PM
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peteTLM

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Whats the caravan heating? Are these summer holiday lets or selective all year round occupancy? Im with andy on the idea of a looped/ resin jointed large cable bus with 63a behind it

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 22 January 2013 09:39 PM
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andyedtec

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Originally posted by: AJJewsbury

You won't get much discrimination between the 16A RCBOs and 32A MCBs on faults (L-N on the TT section and both L-N & L-PE in the TN section) - not sure if that's acceptable or not.



What's the layout like? Would running a single main direct from the BS 88, looping to each hookup, work? (possibly larger cable, but fewer of them and avoiding the cost of a DB, and giving better resilience from a fault in one pitch affecting its neighbour and maybe better use of diversity. Downside would be that all pitches would go off if the main was damaged - but presumably that's a lot less likely that a fault within a pitch. Just a thought.



- Andy.


Thanks for your thoughts Andy.

I had thought about that method but when I worked it out I was getting a 25mm Cable, A concern is the terminals only take 25mm terminals.
And the ground work would double.

The cost of the 6 Way Crabtree Dis board is about 39quid so nothing really.

If the 32A where type C I should have better discrimination and shouln't really affect the max zs becuase it's on TT.?
 22 January 2013 09:44 PM
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andyedtec

Posts: 279
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Originally posted by: peteTLM

Whats the caravan heating? Are these summer holiday lets or selective all year round occupancy? Im with andy on the idea of a looped/ resin jointed large cable bus with 63a behind it


These are only pitches for touring caravans.
 22 January 2013 09:52 PM
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AJJewsbury

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If the 32A where type C I should have better discrimination and shouln't really affect the max zs becuase it's on TT.?

Not really. You need the 32A to trip if there was a fault (L-N or L-E) anywhere up to the hookup and so have loop impedances to suit. A L-N fault a couple of inches further on (just into the TT section) will generate almost the same fault current, so will almost certainly be well within the MCB's tripping range too. Even quite a few feet into the TT section (i.e. pretty much all of the pitch), L-N fault currents won't have diminished much. Remember that MCBs have quite a wide range of possible operating values - e.g. a 32A C type would be between 5x and 10x In - i.e. anywhere between 160A and 320A - so you need loop impedances that'll allow at least 320A to flow, yet you can't be sure it won't trip instantly unless the fault current is below 160A.
- Andy.
 22 January 2013 09:56 PM
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AJJewsbury

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These are only pitches for touring caravans.

How's the electricity to be charged for?

If it's in any way "included in the pitch" price, you'll find people will try to heat the field using fan heaters during the Easter break. If it's metered people seem to be much more restrained.

- Andy.
 22 January 2013 09:58 PM
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andyedtec

Posts: 279
Joined: 28 July 2006

Originally posted by: AJJewsbury

If the 32A where type C I should have better discrimination and shouln't really affect the max zs becuase it's on TT.?


Not really. You need the 32A to trip if there was a fault (L-N or L-E) anywhere up to the hookup and so have loop impedances to suit. A L-N fault a couple of inches further on (just into the TT section) will generate almost the same fault current, so will almost certainly be well within the MCB's tripping range too. Even quite a few feet into the TT section (i.e. pretty much all of the pitch), L-N fault currents won't have diminished much. Remember that MCBs have quite a wide range of possible operating values - e.g. a 32A C type would be between 5x and 10x In - i.e. anywhere between 160A and 320A - so you need loop impedances that'll allow at least 320A to flow, yet you can't be sure it won't trip instantly unless the fault current is below 160A.

- Andy.

I just realized what I said. Tired tonight.
 22 January 2013 10:13 PM
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andyedtec

Posts: 279
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I had offered the metering however it was declined.

The Hookup units I'm using are the Rolec ones and have a mini meter option for about £20, I might fir them anyway.
 23 January 2013 10:24 AM
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Ricicle

Posts: 851
Joined: 23 October 2006

Originally posted by: cmatheson

I prefer a single rcd serving the entire site behind a lock (or screwed panel as well as rcbos for individual final circuits) so that, in the event of a fault, the responsible person will be made aware.


That would be a poor design from a nuisance tripping point of view. It is also not permitted by BS7671. (708.553.1.13)

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 23 January 2013 10:51 AM
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AJJewsbury

Posts: 11786
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A concern is the terminals only take 25mm terminals.

Can the hookup accommodate a few extra DIN rail terminals to loop into? Or as Pete suggested, Tee off outside the hookup.

when I worked it out I was getting a 25mm Cable

What was the limiting factor? If it was v.d. did you consider that the load will decline with distance, so only the 1st section of the cable will be carrying the full load and so on? If Zs, did you consider a time-delayed RCD at the supply end?

And the ground work would double.

You've had me puzzling over that! Given that the single SWA could follow the same trenching as your 3 originals (even if it meant it was centre-fed, or even tree shaped), I haven't yet worked out why it means any more ground work.

- Andy.
 23 January 2013 08:47 PM
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andyedtec

Posts: 279
Joined: 28 July 2006

Originally posted by: AJJewsbury

A concern is the terminals only take 25mm terminals.


Can the hookup accommodate a few extra DIN rail terminals to loop into? Or as Pete suggested, Tee off outside the hookup.



when I worked it out I was getting a 25mm Cable


What was the limiting factor? If it was v.d. did you consider that the load will decline with distance, so only the 1st section of the cable will be carrying the full load and so on? If Zs, did you consider a time-delayed RCD at the supply end?



And the ground work would double.


You've had me puzzling over that! Given that the single SWA could follow the same trenching as your 3 originals (even if it meant it was centre-fed, or even tree shaped), I haven't yet worked out why it means any more ground work.



- Andy.


I may be able to do that, I haven't seen the hook ups yet.

I had looked at that option.

Andy your right I was so tired last night I didn't know what I was typing.

I Have given it some thought, I have considered the discrimination between the 32a and16a, I don't think it would be that big a problem if it did take out the 32a mcb in event of faults you mentioned, at least this would mean only the owner of the site could reset the fault if it did happen.

Thanks for all you comments, I probably would do it differently in the future, but on this occasion the spare ways in the Dis Board will come in handy in the future.

Andy

Edited: 23 January 2013 at 08:57 PM by andyedtec
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