Sub Mains

[Duane B]

Hi guys & Gas

Been to a job today blown fuse, no probs rectified it, however did notice the 3 phase board, 

Meter tails each Henley blocked and feeding 3 x kmf type switchs Un named 100A each rated sealed didn't look DP, these then feed 3 x 25mm 3 core swa which then appears to feed 3 x shops so a further 3 consumer units 1 per shop.  Is this allowed not seen this setup before.  Isn't the feeds to these submain supposed to be rcd rcbo etc at front end  oh and no meters either in the shops so 3 phase end doing all metering. 

Like I said only got called for powercut. 

 

[Andy™]

why would the submains need RCD protection?

 

[ProDave]

The only issue I can see is a contractual one of how does the landlord know how much to charge each shop for their electricity?  the 3 phase total divided by three does not sound very fair if one is a baker and another a newsagent for instance.

 

[Andy™]

the newsagent probably has 20 people living in the back...

 

[Duane B]

3 x High street shop's 1 x news agent mot station at rear feeding it all off there 3 phase,

. Lol but yeah did notice flats above shops ha ha ha, well guessed guys, I always thought sub mains  had to be protected front end obvious you can't get 25mm in an mcb breaker so Henley it, I get that just don't seem right, just thought why not 16mm straight out 3 phase board breakers, 

First time I'd seen a set up this way, learning everyday

 

[Davethsparky]

3 x High street shop's 1 x news agent mot station at rear feeding it all off there 3 phase,

. Lol but yeah did notice flats above shops ha ha ha, well guessed guys, I always thought sub mains  had to be protected front end obvious you can't get 25mm in an mcb breaker so Henley it, I get that just don't seem right, just thought why not 16mm straight out 3 phase board breakers, 

First time I'd seen a set up this way, learning everyday

No you should not feed a distribution circuit from an MCB as you will not achieve any discrimination under fault conditions. This is especially important in a non-domestic installation.

the distribution circuits do have to be protected, yes, but you said they are all fed via KMF switch fuses so they obviously do have a suitable ocpd installed. 

And 25mm will fit into an MCB, 

 

[Evans Electric]

Think about it Ampsyaman ,  surface clipped SWA sub mains wouldn't need an arseydee  , unless you decided to bury them, less than 50mm deep, in the wall .  Now THAT would be a rarity .....a 25mm X 3core SWA chased into a wall .

Also  I'm just saying here that there is a definite general misunderstanding on where an  RCD should go , its not on on sub mains.

And I quoted a job we did years ago now , Barclays Bank in Bedworth I think , with a 200A  TP RCD as a main switch ......big mistake .  

 

[Andy™]

you should not feed a distribution circuit from an MCB

:slap

 

[NozSpark]

Think about it Ampsyaman ,  surface clipped SWA sub mains wouldn't need an arseydee  , unless you decided to bury them, less than 50mm deep, in the wall .  Now THAT would be a rarity .....a 25mm X 3core SWA chased into a wall .

Also  I'm just saying here that there is a definite general misunderstanding on where an  RCD should go , its not on on sub mains.

And I quoted a job we did years ago now , Barclays Bank in Bedworth I think , with a 200A  TP RCD as a main switch ......big mistake .  

Surely if you burried SWA in a wall then it wouldn't need RCD protection as it has "in built" mechanical protection in the armour??

 

[Evans Electric]

Ah  ! Nice one  Noz ..of course you're correct there,  I didn't think that one through .

 

[Davethsparky]

:slap

Whats the problem? A distribution circuit should not be fed from an MCB as it will not achieve full discrimination.

 

[NozSpark]

Whats the problem? A distribution circuit should not be fed from an MCB as it will not achieve full discrimination.

Really??? Where's this reg then???

Wouldn't a D32 and a B32 have discrimination then???

 

[Davethsparky]

Really??? Where's this reg then???

Wouldn't a D32 and a B32 have discrimination then???

What reg, the one which requires discrimination between protective device in series? Sheer common sense is enough isn't it? Section 536 refers to discrimination. Not providing discrimination between the protection of a distribution circuit and the final circuits it feeds is just plain ridiculous.

No a D32 and B32 won't fully discriminate under fault conditions.

 

[Davethsparky]

Really??? Where's this reg then???

Wouldn't a D32 and a B32 have discrimination then???

As you can see from this discrimination table for Schneider iC60 mcbs (the old merlin gerin stuff) no discrimination occurs when a D32 is place in series with a B32, only a 0.5A or 1A MCB will fully discriminate with a D32.

 

[Sidewinder]

The Schneider software is more accurate & easier to visualise the result, total,discrimination can be achieved, but, not often, & not without some thought.

However, it may, or may not be an essential part of the design.