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I see in the UK that RCBO's are more becoming a standard installation now.

I might be missing something obvious here but I've been wondering why are the commonly used RCBO's rated to 30mA leakage current when they're only protecting a single final circuit. Surely 30mA leakage on a normal domestic circuit is astronomically high especially when viewed through the lens of the whole installation having to be above a Megaohm in IR value which equates to 0.2 miliamps of global leakage. Why don't 10mA RCBO's become the norm for a single circuit? I know 30mA is the value of shock current that ought not to cause death but I'm sure anyone who's had a whack from a circuit fed by a 30mA rated protective would agree that limiting that current to a lower value would result in a significantly safer installation for the user.
 
I guess if there's a dozen sockets or so on a ring circuit it might cause an annoyance tripping issue with functional leakage but even with the older arrangement of the dual RCD CU, two or three RFC's would be supplied by a 30mA RCD and it rarely resulted in a problem?
 
I see in the UK that RCBO's are more becoming a standard installation now.

I might be missing something obvious here but I've been wondering why are the commonly used RCBO's rated to 30mA leakage current when they're only protecting a single final circuit. Surely 30mA leakage on a normal domestic circuit is astronomically high especially when viewed through the lens of the whole installation having to be above a Megaohm in IR value which equates to 0.2 miliamps of global leakage. Why don't 10mA RCBO's become the norm for a single circuit? I know 30mA is the value of shock current that ought not to cause death but I'm sure anyone who's had a whack from a circuit fed by a 30mA rated protective would agree that limiting that current to a lower value would result in a significantly safer installation for the user.
It’s all down to the touch voltage, the regs would have to be rewritte, can you imagine
 
the regs would have to be rewritte, can you imagine..........

......... all that extra income for iet, niciec,napit, brexit, covid, NHS, RSPCA, Save the Children in yemen, nigeria, rest of africa, £500 a month should cover the lot.
 
Why don't 10mA RCBO's become the norm for a single circuit? I know 30mA is the value of shock current that ought not to cause death but I'm sure anyone who's had a whack from a circuit fed by a 30mA rated protective would agree that limiting that current to a lower value would result in a significantly safer installation for the user.
You have to remember that RCD do not limit the shock current as such.

What they limit is the exposure time above the trip current.

So in reality if you touch some L-E setup your perception and pain comes down to how conductive you are and how quickly the RCD disconnects. Given that most RCD/RCBO that are not of the delay type go in similar times (10-30ms usually measured) there are only a few cases when a 10mA RCD is going to help you much.

You do get them, typically for one-off RCD sockets in the UK, and they are sometimes seen in areas with a high risk of a shock, but I doubt they really make much difference to the outcome over a 30mA one. Added to that is the UK practice of having dozens of sockets off a 32A ring so when they are often adding the odd 0.5-2mA of leakage due to noise filter capacitors, etc, you can still have a decent number of appliances and not be over the recommended 1/3 RCD rating (i.e. 10mA with a 30mA threshold) to keep nuisance trips down.
 
Be nice at next test time.

“Sorry, that new consumer unit I fitted last year is now all C2 as the rcbo’s need replaced.”

*depends how any new reg is worded of course
 
...... but I'm sure anyone who's had a whack from a circuit fed by a 30mA rated protective would agree that limiting that current to a lower value would result in a significantly safer installation for the user.
I am not sure what you mean by limiting the current, an rcd/rcbo doesn't limit the current.

If you have a resistance of 230 ohm and touch between line and earth/cpc then 1A will flow irrespective of what the protection device is set to.

A 300mA, 30mA, or 10mA rcd will all operate in about the same time for this fault, so from a direct touch perspective there is no difference.

The only difference comes when there is a leakage current, the leakage current could raise the touch voltage for non-direct to ~50V which is tolerable.

The leakage current can be quite high, not because of the insulation resistance, but things like surge suppressors in equipment (small surge capacitors in delta or vee L-E-N ), or general leakage in equipment, this quickly mounts up with computers and similar supplies (just think a typical home computer is likely to have the pc, monitor, printer, possibly storage etc).

10mA is likely to cause random trips on many socket circuits.
 
You have to remember that RCD do not limit the shock current as such.

What they limit is the exposure time above the trip current.

So in reality if you touch some L-E setup your perception and pain comes down to how conductive you are and how quickly the RCD disconnects. Given that most RCD/RCBO that are not of the delay type go in similar times (10-30ms usually measured) there are only a few cases when a 10mA RCD is going to help you much.

You do get them, typically for one-off RCD sockets in the UK, and they are sometimes seen in areas with a high risk of a shock, but I doubt they really make much difference to the outcome over a 30mA one. Added to that is the UK practice of having dozens of sockets off a 32A ring so when they are often adding the odd 0.5-2mA of leakage due to noise filter capacitors, etc, you can still have a decent number of appliances and not be over the recommended 1/3 RCD rating (i.e. 10mA with a 30mA threshold) to keep nuisance trips down.
Bit of a cross post/first to the submit button there!

"Great minds..." and all that....

(Cue someone to add "fools seldom....") :)
 
If you want to know why 30mA and not 100mA or higher (as seen for TT incomers) then it follows (along with some of the time limits) from the 'AC3' region of shock effects:

What that also shows is even with a 10-20ms disconnection time you could still be killed if the current is high enough, hence the rules of ELV in bathrooms in contact with water, etc, where you could be sufficiently conductive to get ampere-level shocks.
 
I am not sure what you mean by limiting the current, an rcd/rcbo doesn't limit the current.

If you have a resistance of 230 ohm and touch between line and earth/cpc then 1A will flow irrespective of what the protection device is set to.

A 300mA, 30mA, or 10mA rcd will all operate in about the same time for this fault, so from a direct touch perspective there is no difference.

The only difference comes when there is a leakage current, the leakage current could raise the touch voltage for non-direct to ~50V which is tolerable.

The leakage current can be quite high, not because of the insulation resistance, but things like surge suppressors in equipment (small surge capacitors in delta or vee L-E-N ), or general leakage in equipment, this quickly mounts up with computers and similar supplies (just think a typical home computer is likely to have the pc, monitor, printer, possibly storage etc).

10mA is likely to cause random trips on many socket circuits.
My thinking is to reduce the shock duration for someone who contacts a live part of the installation.

If an average person fully dressed and not wet has a resistance in the region of 10kiloohms contacts a live part at a voltage of 230v then the shock current will be 23mA. This fault current alone is insufficient to trip a 30mA RCD or RCBO. In reality the resistance of the person may be a bit less than 10kilohms and there would probably be some background leakage which would cause the RCD/RCBO to trip but it would probably be slow. If the device was rated at 10mA it would mean any person contacting any live part would definately initiate a trip and achieve a predictable disconnect time.

I'm interested why 10mA would be likely to cause random trips on many socket circuits. Functional leakage should be a couple of mA on a circuit and if there's even 1 mA leakage due to insulation it means that circuits IR value is way under 1 meg which would be considered a fault.
 
My thinking is to reduce the shock duration for someone who contacts a live part of the installation.

If an average person fully dressed and not wet has a resistance in the region of 10kiloohms contacts a live part at a voltage of 230v then the shock current will be 23mA. This fault current alone is insufficient to trip a 30mA RCD or RCBO. In reality the resistance of the person may be a bit less than 10kilohms and there would probably be some background leakage which would cause the RCD/RCBO to trip but it would probably be slow. If the device was rated at 10mA it would mean any person contacting any live part would definately initiate a trip and achieve a predictable disconnect time.

I'm interested why 10mA would be likely to cause random trips on many socket circuits. Functional leakage should be a couple of mA on a circuit and if there's even 1 mA leakage due to insulation it means that circuits IR value is way under 1 meg which would be considered a fault.
Even tripping within 40ms, a typical person would still be within the AC2 range for up to circ 150mA. So the difference between 10mA and 30mA with trip times within 500mS is still within AC2.


As I stated previously, it is not insulation resistance, but leakage within the devices, there are maximum limits specified for each piece of equipment as follows:
  • class II appliances and for parts of class II construction - 0.25 mA.
  • For class 0, 0I and III appliances: 0.5 mA b for class I portable appliances -, 0.75 mA.
  • For class I fixed motor-operated appliances - 3.5 mA.
  • For class I fixed heating appliances - 0.75 mA or 0.75 mA/kW of rated power, with a maximum of 5 mA, whichever is the higher.
IEC 60335 series provides:
  • Fixed PC workstation - 2 mA
  • Printer - 1 mA
  • Photocopier - 1.5 mA
  • Laptop - 0.5 mA (with EMC filter)
  • Grills, toasters/portable cooking appliances - 0.75 mA (earthed metal)
  • Fridges - 1.5 mA (class I)
  • Dishwasher - 5 mA
  • Hobs, ovens - 1 mA or 1 mA/kW of rated power
  • Washing machine - 5 mA
  • Tumble dryer – 5 mA
  • Electric heat pumps - 10 mA (accessible to public)
  • Floor heating - 0.75 mA or 0.75 mA/kW of rated power
BS EN 60598-1 give leakage current for Luminaires:
  • Continuous interference - 0.5 mA
  • Class 0 and Class II -1 mA
  • Portable, Class I - 1 mA
  • Fixed, Class I up to 1 kVA of rated power, Increasing in steps of 1 mA/kVA up to a maximum of 5 mA
So a typical kitchen or utility room circuit having a washing machine and dishwasher hits 10mA between them, ~4 computers would also hit 10mA, ( made up of PCs, printers etc)

Appliance leakage is by far the biggest reason for earth fault leakage and quickly adds up.

30mA is a reasonable balance between limiting impact of direct contact, and nuisance tripping.

A move to 10mA would mean that circuits would have to be further divided, a typical kitchen would have to have 4-5 radials, one each for dishwasher, washing machine, dryer, then a couple for general kettles/microwave/toaster/etc plus the cooker circuit.

Similarly many other rooms would need to be sub-divided, any lighting circuit could cover no more than 10 luminaires, so with many modern lighting arrangements, this could be insufficient for one fancy room, assuming we need a rcbo, plus afd for each circuit, I dare to think how big the cu would need to be.
 
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My thinking is to reduce the shock duration for someone who contacts a live part of the installation.
Several posters have already mentioned that shock duration is unlikely to be any different, whether using 10 or 30 ma rcd, s in situations where the fault currant is above 30 ma. I agree with that conclusion. However one area where a 10 ma rcd may well make a difference is in situations where fault current is below 30 ma. Let's say an individual receives a significant shock (say 25 milli amps) that is below the 30ma threshold but above the "no let go" threshold. The Electrical Safety Council had a number of very informative video,s of individuals who were literally paralysed as a result and stranded in a very precarious situation. I understand that 10 ma rcd, s are quite common In The USA
 
Several posters have already mentioned that shock duration is unlikely to be any different, whether using 10 or 30 ma rcd, s in situations where the fault currant is above 30 ma. I agree with that conclusion. However one area where a 10 ma rcd may well make a difference is in situations where fault current is below 30 ma. Let's say an individual receives a significant shock (say 25 milli amps) that is below the 30ma threshold but above the "no let go" threshold. The Electrical Safety Council had a number of very informative video,s of individuals who were literally paralysed as a result and stranded in a very precarious situation. I understand that 10 ma rcd, s are quite common In The USA

I thought that threshold was in the region of 30mA?
 
I hear the point about cumulative leakage within the devices possibly being sufficient to cause nuisance tripping but I think there's a purely logical argument for 10mA RCBO's where history has shown that an entire installation can be free from nuisance tripping with a dual 30mA RCD setup or at least the socket circuits can be supplied from a single 30mA RCD without nuisance tripping due to functional earth leakage. So logically it shouldn't make a difference if a domestic installation with 3 socket ccts, 2 lighting circuits and a miscellaneous circuit for a fixed appliance or garage is supplied by 2x 30mA RCD's or 6x 10mA RCBO's.
 
Let's say an individual receives a significant shock (say 25 milli amps) that is below the 30ma threshold but above the "no let go" threshold. The Electrical Safety Council had a number of very informative video,s of individuals who were literally paralysed as a result and stranded in a very precarious situation.
Were they on RCD that did not trip?

Remember the RCD is supposed to be "additional protection" so you should already be seeing ADS on a fault to earth, etc. Then we are looking at a small window from, say, 20mA "no let go" to 30mA for upper RCD threshold and I would expect in the case of being unable to let go that you actually grip harder, causing the current to increase and probably trip the RCD then.
I understand that 10 ma rcd, s are quite common In The USA
I think they are often on the outlets though, so only one or two things plugged in (rather like our recently-depreciated RCD sockets / FCU).
 
Were they on RCD that did not trip?

Remember the RCD is supposed to be "additional protection" so you should already be seeing ADS on a fault to earth, etc. Then we are looking at a small window from, say, 20mA "no let go" to 30mA for upper RCD threshold and I would expect in the case of being unable to let go that you actually grip harder, causing the current to increase and probably trip the RCD then.

I think they are often on the outlets though, so only one or two things plugged in (rather like our recently-depreciated RCD sockets / FCU).
These were all cases studies put forward by the ESC in their quest to gain more acceptance of rcd, s in the early noughties. One particular example had to do with a housewife cleaning a damaged appliance lead with a damp cloth. She remained paralysed and unable to move until discovered by a family member.
 

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