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Hi,

Regarding discrimination requirements in regs..

If a second cu is fitted in a shed or outbuilding supplied by an rcd protected way at the main cu then it provides protection at a change of cable size due to the mcbs but that leaves two rcds in series with no discrimination. However is this a problem? I see several options each with advantages and disadvantages:

1 rcd at both supply and in outbuilding

- more cost, no discrimanation,

+redundancy/diversity in protection
Potential for local rcd to trip and this may be better than having rcd solely at the main cu as there is chance local fault would trip local cu rather than main cu
Supplu cable rcd protected (although not strictly required if selected /installed correctly)

2 supply fed from non rcd protected supply with local rcd protected cu in outbuding

+good discrimination
- no rcd protection of supply cable. (however not required if selection/installation of supply cable is compliant)

3 fed from rcd protected supply in main cu with non rcd protected cu in outbuilding with appropriately sized mcbs to provide overcurrent protection

+ potentially cheaper, supply cable protected (although again not strictly required)

- if rcd trips depending on main cu could take other circuits out, at the very least requires going back to main cu to reset in event of spurious trip

Looking at the options I kind of prefer option 1 but it's extra cost but has some maybe marginal benefits. Any thoughts?
 
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2 is the best option provided the cable to the shed does not require additional rcd protection or an rcd for fault protection. 3 is an alternative but 1 does not ensure selectivity. (discrimination is an old term).
 

Wilko

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Arms
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Hi Dan - At my house I inherited a 32A distribution circuit to the garage which had a small CU with an upfront RCD. At some point they’d upgraded the house to twin RCD CU so the final circuits in the garage ended up with 2 RCDs in series. Wilko moves in and says “I need an electric door opener - make it so”. Some time passes and I do it myself ... Then one night the DNO supply does a few backflips (going to blame them anyway) and the garage CU RCD trips. This can’t be reset from outside the garage and I couldn’t find the garage side door key. Ho hum. So my electric fix was to remove the garage CU RCD and replace it with an isolating switch.
 

plugsandsparks

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Arms
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Reminds me of a job a few years back, got a call " Breaker to garage tripped cannot reset it and only way in is electric door" -
Sure enough 32A breaker in house to garage would not reset.
I explained the problem that we may have to break in etc and client mentioned a small hatch between the house and garage, only wide enough for half of P&S... lol - Anyway i looked in with a torch and saw two 2 KW heaters plugged into one extn reel..... eeek me thinks , using various long rods etc i managed to flick the plug out of the socket and hey presto, breaker reset, now in the garage and looked at the extn reel which was a molten blob of plastic and copper..... a proper destruction job... lol - anyway after a brief sermon about the issues with extn reels and 4KW of heating, i was on my way.....
 

Pete999

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Arms
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Hi,

Regarding discrimination requirements in regs..

If a second cu is fitted in a shed or outbuilding supplied by an rcd protected way at the main cu then it provides protection at a change of cable size due to the mcbs but that leaves two rcds in series with no discrimination. However is this a problem? I see several options each with advantages and disadvantages:

1 rcd at both supply and in outbuilding

- more cost, no discrimanation,

+redundancy/diversity in protection
Potential for local rcd to trip and this may be better than having rcd solely at the main cu as there is chance local fault would trip local cu rather than main cu
Supplu cable rcd protected (although not strictly required if selected /installed correctly)

2 supply fed from non rcd protected supply with local rcd protected cu in outbuding

+good discrimination
- no rcd protection of supply cable. (however not required if selection/installation of supply cable is compliant)

3 fed from rcd protected supply in main cu with non rcd protected cu in outbuilding with appropriately sized mcbs to provide overcurrent protection

+ potentially cheaper, supply cable protected (although again not strictly required)

- if rcd trips depending on main cu could take other circuits out, at the very least requires going back to main cu to reset in event of spurious trip

Looking at the options I kind of prefer option 1 but it's extra cost but has some maybe marginal benefits. Any thoughts?
Do you mean in Parallel? as RCDS require a N to work properly, or at all
 
  • Thread Starter Thread Starter
  • #7
Yes in reality the wiring strictly is in parralell but the rcds figuratively are in series...
 
  • Thread Starter Thread Starter
  • #10
Getting myself confused here... by series I meant that there's two on the circuit. They operate in parralell as either one could trip in the event of a fault
 

Pete999

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Arms
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davesparks

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Mentor
Arms
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Do you mean in Parallel? as RCDS require a N to work properly, or at all
I would say they are in series, not in parallel.
When connected in series all of the load current passes through each device, when in parallel the load current splits between the devices in proportion to their impedances.
 
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