Outhouse Supply Debate

The 40 amps of the rcd is it's total load capacity, nothing to do with tripping. An rcd doesn't respond to overload or short circuits, it only trips on earth faults, ie, n-e or l-e.

 

It's pointless using the same breaker size at the start and the end of the run. Your meant to provide at least half the protection size each time there is a reduction in protection (ie cable size), so 32a at the CU, then 16a/6a at the shed CU. It's to help with discrimation. There may be a reg some where.

 

I would keep quiet about being a sparks

 

As an aside, if a the house fuse is 100 amp and you want a long run to a consumer unit, some members say you need to put in a 60 amp switch fuse at the start of the run, which, in theory, would provide discrimation, but if you look at time/current characteristics the disconection time between a 100a and a 60a fuse is so minimal as to be pointless, ie both would blow at the same time in the event of a fault.

 

I'm not 100% sure what your getting at mate, but if you mean what rating the equipment needs to be, then the RCD would need to be rated at 32a or greater as it would need to withstand a current of at least 32a due to the 32a breaker in the house CU. So a 40a RCD, in this case, would be more than enough.

 

I think your confusing RCD's and breaker mate.

 

If the shed wasn't going to have an RCD, would you be trying to look for a 32amp main switch? Would you not be happy using a 100amp main switch?

When you install a 32amp circuit for a 32amp machine, do you implicitly look for a 32amp rotary isolator, or would you be happy with a 40amp? (incidentally, recently, I installed a 10amp circuit using 16mm SWA due to length, and used a 63amp rotary isolator for room).

The 40amp on the RCD is just it's maximum rating it can happily carry/break. This rating needs to be equal to or greater than the 32amp protection at the origin. I would be happy if it was 32, 40, 63 or a 100. 40amp are a very common size. You would struggle to find a 32.

I think on this occasion you need to bow down to your mate
Although, ask him why he is using the RCD protected side at the origin when there is no need. The SWA doesn't require RCD protection, and having two RCD's on the same circuit when one is not a time delayed type, provides no discrimination. RCD at the shed is all that is required.

 

I always thought industrial electrics was far more involved then domestic/cabling up a humble shed type electrics - although only diy myself I am surprised at the questions Staffsspark is asking

 

It's the difference between full time maintenance electricians and jobbing it sparks.

 

BS EN 60898 circuit breakers in series are not going to provide discrimination anyway, regardless of rating. (Nothing to do with original question, but someone else alluded to this.)

 

jusy wondering why there is going to be two RCDs in series?

 

My caravan has a 30mA RCD and a 16A trip for the sockets and a 6A for the lights yet every caravan site has a 30mA RCD as 16A MCB so in theroy there is no point in the 16A trip or RCD in the caravan but the IET/BSi regulations still say I must have a RCD in the caravan.

Yes having a 10A MCB for sockets would be better than my trip will trip first. Although debatable which is the easiest to reset. Also having the consumer unit under the bed is not really the best place specially since the 12 volt power supply is built into the same unit. However that's where the caravan manufacturer put it and how they designed it.

The shed is similar I would agree 30mA RCD in house and shed seem pointless but should the consumer unit in the house be changed having the shed already with RCD fitted means at that point the shed can be supplied from an unprotected bus bar.

As to 32A MCB yes agreed pointless but you need something to connect to.

In theroy you could simply wire the shed as a lollipop ring with a switched FCU with 3A fuse for the lights there is no need for a consumer unit. I have wired many containers used as workshops in that manor but there is not much room for 6mm SWA and two 2.5mm in the same double socket box. Using a consumer unit makes it easy to terminate and allows a change in the future.

It's more down to convenience than requirements, the consumer unit likely comes with a 32A, and 6A MCB and a 40A 30mA RCD it is not worth changing. We would hope neither will ever trip anyway only if they do would one consider changing them from standard.

I suppose a CT transformer in the caravan and an ammeter would also help not to over load but the times is trips are few and far between so why bother.

 

The current carrying capacity rating of devices is a maximum. They don't have to match downstream devices but they must be at least the same or greater. The current carrying capacity rating is the capacity of that device to carry that current. This is completely different to the rating of the device to protect a circuit from overcurrent or short circuit. I would have thought this should be obvious but clearly it isn't. 100A switch is rated to carry 100A. 63A 30mA RCD is rated to carry 63A but trip at 30mA tripping current (there is no overload or short circuit protection). A fuse or mcb rated at 32A is designed to break the supply to that circuit if it exceeds 32A or there is a short circuit of negligible impedance. The protective rating of the device to break that current is 32A, but it may have a 6kA or 10kA rupture capacity to withstand a fault without exploding.

 

Can't help but wonder how an "electrician" cannot know or understand this UP!

 

Can't make it any simpler.