Discussion in 'Electricians' Talk' started by furious_customer, Mar 13, 2018.
28+vat for 10 way board & 6 rcbos @ 8.50 +vat =94.80 inc from your friendly CGD stockist
So stupid questions of the day...
A standard CU has 2 x RCD's - is this one for the kitchen and one for the bathroom?
How is the overcurrent protection achieved for these circuits?
And so RCBO's are combined RCD's and breakers - can/should these be fitted to every circuit in a house, including lights?
I have heard of some people having problems with high current applicances like washing machines causing he RCD to trip unnecessarily - why is this?
No, 2 RCDs feeding two "sets" of MCBs for the various circuits. Like this
Some CUs have 1 or 2 slots direct off the main switch that aren't RCD protected, can place an RCBO in there for the fridge circuit , for example.
Alternative is not to have RCDs, but all circuits are on RCBOs (combining RCD and MCB).
Ah ok - so one RCD can monitor more than one MCB?
Aye these units are called high integrity units Doc - will never understand why people would not buy this type as it is very handy for whipping in rcbo/s for lets say exterior lighting/fridge freezer and stuff..
So why would I want to have an RCBO rather than and RCD monitoring a bunch of MCB's?
Oh - and also wondering why there are 2 RCD's rather than just 1?
Yes exactly. My CU has a 63A and 80A RCD, typically you would put ground floor ring and first floor lights on one, first floor ring and ground floor lights on the other. Additional circuits like oven are put on one or other RCD so its fairly balanced.
RCBO's have advantages over a MCB, they offer overcurrent protection and earth fault protection in one module, & with a RCBO only one circuit is affected if it trips, not multiple circuits with RCD.
If there's a fault on one circuit, it'll trip the RCD which will disable all other circuits that its on, half the house. With RCBO, each individual circuit is essentially on its own RCD so everything is separate. Its a better design but more expensive.
2 RCDs mean not all eggs in one basket.
Great - thanks!
My CU has a total of 40 positions.
The incomers feed three subsets.
Set 1. A dual pole switch/isolator which in turn feeds a series of RCBOs that protect specific circuits. HiFi, Fridge/Freezer, Comms Room, Garden, Intelligent lighting system, &c. So, if one of those has a fault and trips, the others are maintained.
Set 2. A 30mA RCD feeding series of power ring finals and radials. If any of these goes over current the MCB will trip leaving the remainder operation, however if a fault occurs and the RCD trips all circuits will be de-energised/OFF
Set 3. A 30mA RCD feeding a second set of power rings and finals.
Compared to most domestic installations, I think a 40-module board is slightly more...erm...overkill!
That being said, there are benefits to splitting off circuits into smaller areas/uses. Application specific.
I agree, 40 way is a bit OTT for most people, mine is an MK 21 way, up to 2 slots non-RCD protected which will be the RCBO for fridge/freezer and cabin SWA.
It is not the 'high current' that will cause the RCD to trip. It is a high 'leakage current', that is a current over 30mA flowing to earth, that cause the RCB to operate. Cookers and kitchen appliances are common sources of faults that cause RCDs to trip.
If you have a spare twenty minutes this John Ward video will explain all.
Thanks - I was actually 3/4 way through JW's video - which is what made me wonder and ask the question in the first place.
I need to read-up more on leakage current to understand what causes it - e.g. is it always due to a fault? Or is it just a property of some devices?
Another question - if I chopped through a live-wire on an RCD circuit with an un-insulated pair of cutters - would I feel *anything* at all - or is the RCD so fast that it will cut the supply before I even know about it?
Have just counted - actually 2 linked16 module units. There are a few spare ways, and maybe a little over the top but it does what is needed. 4 bed, 2 bath, 3 utility/stores ... with two original lighting rings - now almost redundant as everything is moved to an intelligent system, plus dedicated circuits, soon increases the number of modules required. The summer house/workshop and garage feeds are also separate!
And having te space to put it, made it an easy choice to have a several dedicated rings or radials.
Interested to see you Max demand workings out
In almost all household appliances it will be a fault usually caused by the breakdown of insulation or water getting to parts where it shouldn't be. There are many devices that are designed to allow very small currents to flow to earth eg some power supplies in electronic equipment. Not usually enough to trouble an RCD unless you have lots on the same circuit eg a server room.
(If this answer is wrong I accept evidence from those who know different from experience)
It would depend upon whether you were also touching something which was earthed at the same time whether you a) got a shock and b) whether the rcd would trip.
Exactly right. It would depend on the resistance between the live and earth i.e. through the person.
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