Correct discrimination?

[leokiss]

Id like to run a 20m 4mm2 armoured to a log cabin/shed type thing!? On a 32amp mcb on an rcd C/U
Then have small garage type C/U in cabin but they always seem to come with 63 or 40amp rcds as main switch.
Not sure where i stand with discrimination. Just a few sockets and lights in cabin so a 16 or 20 for sockets and 6amp for lights

 

[truckster]

Mcb discrimination is ok 32A-16A-6A

You will either have to take the swa from a non rcd source(preferred) or an s-type rcd to achieve discrimination on the RCD's

 

[telectrix]

first, the 40A or 63A rating on a RCD is the max. load. nothing to do with tripping. secondly, it's not a good idea to have 2 RCD in series. replace the RCD in the sub-main with a 2 pole isolator feeding your 16A and 6A MCBs, or feed from the house independent of the house RCD, which may entail splitting the tails to the CU.and fitting a separate MCB to protect the SWA.

 

[leokiss]

Mmm thats the problem... what if main rcd for c/u in house is 80 or 100amp would a 63 or 80amp main rcd in cabin be ok?

 

[leokiss]

Oops wrote before last post looks like im going to have to split tails then.
What if the cabin main rcd was swapped for a 100 isolater,cabin would be rcd protected from house main rcd,not ideal but is it legal?

 

[telectrix]

hang on. are you talking amps here or milliamps? the first is the load capacity and the second the trip current . and you don't want 2 rcds. if you are supplying via the house rcd, ditch the garage rcd.

 

[leokiss]

Sorry i think i repeated what you said first ,ditching rcd in cabin for isolater,wasnt sure if this is ok thought cabin would need its own rcd but then would end up with them in series

 

[telectrix]

oops, posts crossed.it.s not a problem to have the rcd in the house, but just an inconvenience -if a fault occured in tghe garage, it would take out all circuits on the house rcd.

 

[i=p/u]

right guys you aint reading the post correctly. the cu in the shed/cabin has a 63A main switch and he is asking how will this be with him feeding the shed/caabing cu with a 32A mcb

 

[i=p/u]

does he need a main switch???? of 63A, should this be smaller than 32A??? get with it guys

 

[telectrix]

his main switch can be 1000A for all it matters. it's the MCBs that provide circuit protection. the rating of the main swith is just the current it can safely withstand.

 

[i=p/u]

well i think thats where he getting messed up, and diddnt know it was the current it could stand. am i correct oringinal post poster?????

 

[leokiss]

Ha even im getting confused now, the garage c/u comes with either a 40 or 63 amp rcd feeding a 20 or 16amp and 6amp mcb. The house c/u is a 100amp rcd swich feeding other mcbs in house.I wanted to use a 32amp mcb fed off the 100amp rcd switch. I wasnt sure how i stood with this set up.
But i think we got there in the end by elimating the 63/40 amp rcd switch in garage and changing to standard isolater switch so: 32amp mcb (backed by main c/u rcd) to the garage c/u with isolater to 20amp&6amp mcbs.

 

[i=p/u]

maybe 100mA rcd covering whole installation in house. = s type i think.

anyway you shouldn't have two rcd's in line unless they are of different rataings (I an)

 

[telectrix]

if the house CU has an upfront 100A, 30mA RCD, your problem is that a fault in the garage will take the whole house out. the ideal solution would be a new, hi integrity CU in the house, but cost may be an issue. if that is the case, i would split the tails and feed a 32A MCB in an enclosure, from the henley blocks and use the "garage board" with it's RCD and 2 MCBs. all you need to worry about then is adequate earthing!!!

 

[Engineer54]

anyway you shouldn't have two rcd's in line unless they are of different rataings (I an)

Not even then, unless your talking about 500mA upstream and 30mA downstream. I've even seen that arrangement trip out together, if the leakage has been excessive...
The only sure way of achieving discrimination between RCD devices is by time delay!!

 

[dixon9]

Do you have 3 neutral bars in the house CU? If so then put in a 100mA Time delay S type RCD (bit pricey though) and run the shed/cabin supply mcb off this....you could then use a 30mA RCD in the shed/cabin and have discrimination - should a fault develop in the shed/cabin then only the 30mA RCD in the CU there will trip without taking the house out. The cable run from the house to the shed/cabin is also RCD protected.

 

[leokiss]

I've had a proper look at job today I think I'm going with split tail small enclosure with double pole isolater feeds 32amp mcb which feeds armoured to cabin where I'll fit the small garage type enclosure 63amp 30milliamp rcd switch feeding 20 and 6 amp mcb. Cheapest way and easier of options to install. Cheers for feedback

 

[telectrix]

sounds like a plan. just need to decide whether to use 2 coreSWA using armour as cpc or 3 core. ( or even to TT the garage ). i would deffo use 6mm to allow for possible increase in demand.