IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Sub-Main Isolation/Protection
Topic Summary: Din mounted fuse or switchfuse
Created On: 13 February 2018 09:34 AM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 13 February 2018 09:34 AM
User is offline View Users Profile Print this message



GR1981

Posts: 27
Joined: 29 February 2016

Hi all

I've got a retail job on that needs two sub-mains running off the TP&N board.

In order to get the discrimination, I've opted to use BS88 fuses.

Would it be best to use din rail fuse holders to achieve this, or use a rail mounted terminal block to run to a switchfuse?

I'd appreciate any thoughts (or alternatives even).
 13 February 2018 10:14 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16302
Joined: 13 August 2003

What sort of rating to you need for the submains?

I'd guess you'd typically need something reasonably large (e.g. 63A+) if you've got decent size final circuits downstream and need to discriminate with MCBs (check with MCB manufacturer's data). If so then an MCB sized fuse carrier is unlikely (just because of the physical size of the fuse itself). There are larger sized DIN rail fuse carriers, but they're unlikely to fit a board intended for MCBs (let alone meet the board's type approval etc).

MCB shaped terminals and separate switchfuses are certainly one option - but might take a lot of CCC away from the board - as well as talking up MCB way that might be useful for local circuits in the future.

If space & aesthetics permit I'd consider switchfuses fed directly from the supply - e.g. add a single main switch if there isn't one already and a set of Henley blocks (or a bus-bar chamber if it's a chunkier kind of installation) - which also gives you the advantage of being able to keep some of the installation on if the existing board has to be isolated for any reason.

- Andy.
 13 February 2018 10:21 AM
User is offline View Users Profile Print this message



tattyinengland

Posts: 1009
Joined: 23 November 2006

I'm assuming that you cant use MCCB three phase sub distribution supply at the origin and you have an MCB board at the origin??I'm also assuming you are looking for a cheapest cost option for this job.

Dare I suggest that you install two of, three phase MCBs into the existing DB and supply the sub DBs from there. Discrimination is only ever going to be an issue in the even of a catastrophic fault (Big BOOM) - which is quite unlikely - and then you have to re-set two MCBs - at the origin and at the Sub DB. Do the retail shop tennants have access to the Origin? This may an acceptable small inconvenience.

In order to use BS88s I'd definitely use a whole switched fused isolator at the origin, rather than din rail mounted BS88 fuse holders supplied by singles/tails as they'd have to go into an enclosure of some sort anyway..... . What size supply and sub distribution supply are we taking about?
 13 February 2018 10:47 AM
User is offline View Users Profile Print this message



GR1981

Posts: 27
Joined: 29 February 2016

Thanks for the replies.

One sub-main would be about 25-30A, and the other 35-40A. in both cases i'm accounting for diversity.

the supply given by the landlord is via 100A 3ph switchfuse. The original plan was to take this straight into an MCB board and feed the sub boards from this MCB board.
 13 February 2018 12:50 PM
User is offline View Users Profile Print this message



tattyinengland

Posts: 1009
Joined: 23 November 2006

There are better designers on this site than I that's for sure.............but I'll keep going as I've started.............

I'd be OK with MCBs - its not perhaps ideal - but whats it got to compete with? - a shop as you've described it would surely not have too many large supplies but perhaps the chain of events could go,

1)......By the landlord via 100A 3ph switchfuse
2) 2 of 63 Amp MCBs in a 4 way Three phase DB (maybe you can sell a bigger board to allow for future sub DB supplies) - if your estimate is correct - these should not ever overload...
3) To store 1&2 DBs.

Better yet maybe- at the supply origin install 2 of 3 phase switch fuses

- switch fuses take up a lot of space so make sure you have the space for these.

I'd still stick in 2 x 63Amp sub distribution supplies I think.

Have you considered bonding if required? - We once bought in a new gas supply to a (office to Bar) unit conversion - main bonding was required, sized for the main incoming building supply.
 13 February 2018 02:27 PM
User is offline View Users Profile Print this message



GR1981

Posts: 27
Joined: 29 February 2016

Hi tattyinengland

thanks for the reply. thankfully space isn't an issue. This is quite a large retail unit in a shopping centre. And we have a new earth bar installed for earthing and bonding.

the problem we have with feeding the sub-mains from the MCB's is we get issues with the cut-off of the MCBs.

I think there may be scope to install a small busbar chamber which would make life easier.
 13 February 2018 07:12 PM
User is offline View Users Profile Print this message



Weirdbeard2

Posts: 172
Joined: 29 November 2017

Originally posted by: GR1981





the problem we have with feeding the sub-mains from the MCB's is we get issues with the cut-off of the MCBs.



Hi GR, out of interest what do you find the most popular cause of the MCB cutoff?
 13 February 2018 09:38 PM
User is offline View Users Profile Print this message



hertzal123

Posts: 477
Joined: 26 August 2007

In the past,if there is spare current capacity at the main 3 ph board,I have seen a 100A isolator fitted to one of the outgoing ways in the board and then tails to a switched fuse for the submain.Is this a no no?
Hz
 14 February 2018 10:37 AM
User is offline View Users Profile Print this message



GR1981

Posts: 27
Joined: 29 February 2016

Originally posted by: Weirdbeard2

Originally posted by: GR1981


the problem we have with feeding the sub-mains from the MCB's is we get issues with the cut-off of the MCBs.





Hi GR, out of interest what do you find the most popular cause of the MCB cutoff?



Hi Weirdbeard,

in this case, the peak cut-off of the MCB exceeds the Magnetic / Instantaneous setting of upstream MCB (I'm quoting the calculation package output here).

If I increase the length of the circuit, it seems to clear the issue but I can't really specify that an excess of 20m of cable should be thrown around the place.
 14 February 2018 12:31 PM
User is offline View Users Profile Print this message



davezawadi

Posts: 4018
Joined: 26 June 2002

Lots of cable is not the way to solve this! Upstream BS88s are the way to go, why not a small BS88 DB, although these are getting rarer? You can then choose a rating which is OK for discrimination, at the cost of a slightly increased submain cable size.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2018 FuseTalk Inc. All rights reserved.

..