Discuss I have an RCBO question ???? in the Electrical Forum area at ElectriciansForums.net

Marvo

-
Staff member
Mentor
Esteemed
Arms
Supporter
Reaction score
9,843
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

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.
 
OP
Marvo

Marvo

-
Staff member
Mentor
Esteemed
Arms
Supporter
Reaction score
9,843
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?
 

cliffed

-
Arms
Reaction score
467
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
 

telectrix

-
Mentor
Esteemed
Arms
Reaction score
65,533
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.
 

pc1966

Esteemed
Arms
Reaction score
6,232
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.
 

littlespark

-
Esteemed
Arms
Reaction score
9,881
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
 

Julie.

Esteemed
Arms
Reaction score
2,736
...... 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.
 

Julie.

Esteemed
Arms
Reaction score
2,736
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....") :)
 

pc1966

Esteemed
Arms
Reaction score
6,232
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.
 
OP
Marvo

Marvo

-
Staff member
Mentor
Esteemed
Arms
Supporter
Reaction score
9,843
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.
 

Julie.

Esteemed
Arms
Reaction score
2,736
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.
 
Last edited:

LastManOnline

Esteemed
Arms
Reaction score
851
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
 

nicebutdim

-
Esteemed
Arms
Reaction score
1,984
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?
 
OP
Marvo

Marvo

-
Staff member
Mentor
Esteemed
Arms
Supporter
Reaction score
9,843
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.
 

pc1966

Esteemed
Arms
Reaction score
6,232
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).
 

LastManOnline

Esteemed
Arms
Reaction score
851
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.
 

LastManOnline

Esteemed
Arms
Reaction score
851
From what I gather from posts on here, right up to the point they are removed, shortly after the installation has been inspected!
You may well be right. I had no knowledge of that practice. If correct, that would mark the US electrical culture out as highly unconventional. We all have gripes about some of our respective regs, but its rare to hear of someone willingly breaking them.
 

pc1966

Esteemed
Arms
Reaction score
6,232
You sure about that point? Would the "touch voltage" be any different whether the rcbo rating is 30 Ma or 500 Ma?
Exactly, there are very few situations when you would have a CPC so poor that an RCD won't trip and a substantial voltage exists for touching that might be trip-limited.

It practice you will almost certainly get 0.5-1.0 of the supply voltage for the duration of the RCD trip time, depending on the CPC being full size on TN (so R2 = R1 and 0.5) to practically non-existent (e.g. TT case where 'R2' is very much higher than R1 back to the source so more or less 1.0).
 
Last edited:

Reply to I have an RCBO question ???? in the Electrical Forum area at ElectriciansForums.net

Similar Threads

T
Hi, We are going into business making RCDs. We cannot undertsand why this massive gap in the market has been missed? All existing offtheshelf RCDs are >£30 and massively over-engineered!!!. Our method is a simple version of a "Fluxgate current sensor". That is, we use the fluxgate principle...
Replies
27
Views
1K
Julie.
J
The apprentice
Hi there just have a few questions regarding an agricultural installation. 1. Due to the existing installation being a TN-S earthing supply I understand that regulation 705.415.2.1 (note) that unless a metal grid is laid in the floor (which there is not) the use of a PME is not recommended ...
Replies
3
Views
498
pc1966
P
V
Hi, I've got to put in an additional consumer unit into quite a large property and rather than daisy chaining them back to a single MCB in the main board which is RCD protected, I wondered if this a better solution: The property is served by TNC-S with an isolator switch. Connect new tails into...
Replies
8
Views
233
Matthewd29
Matthewd29
spud1
Hi ! Much talk of the use of type A, type B and Type F RCDs recently. As we know all are specified at varying levels to ignore the blinding effects of transient DC currents, that are being superimposed on our AC circuits by modern technology. But is there a means to measure the level of this...
Replies
1
Views
284
pc1966
P
HappyHippyDad
I need to get 30mA RCD protection to a new circuit I am putting on this consumer unit. I didn't take the cover off so I am not sure if there will be spare ways on the RCD side of it (wish I had checked!). It looks likely that there would be space, does anyone know if the RCD side of this board...
Replies
7
Views
637
telectrix
telectrix

Electrical News from Electricians Forums and Friends

E
  • Article
Want to become a qualified electrician? Look carefully at the training courses on offer to avoid wasting time and money, is the advice in a new campaign from electrical industry body The Electrotechnical Skills Partnership (TESP). With thousands of searches each year on terms such as...
Replies
49
Views
6K
Marty92
Dan
  • Article
You can now link your account with social media by going to the following page. https://www.electriciansforums.net/account/connected-accounts/ We had this feature some time ago but I removed it when Facebook was found to be selling the data they collected without informing the users of what...
Replies
0
Views
2K
Dan
Dan
  • Article
There was a dodgy advert, typical clickbait, showing "how to save 90% on your electricity bill" which was not the case and it was actually showing some dangerous wiring tactics. Read the actual threads here:- What should be the code for this - C2 or C3 -...
Replies
37
Views
8K
Jurassic Sparks
Jurassic Sparks
James
  • Article
The Go Plus hot tub is being recalled because you could get electrocuted taking a dip, according to press releases from the Chartered Trading Standards Institute (CTSI). An Amazon spokesperson said: "Safety is a top priority at Amazon. "We require all products offered in our store to comply...
Replies
6
Views
3K
Dave Appleby
D
Dan
  • Article
Posted on behalf of @CKI:C.K-Tools Carl Kammerling International Ltd, also known to as ‘CKI’, are the company behind the C.K Tools brand. Founded in Germany in 1904 and with roots stretching back to 1790, we provide a wide range of hand tools, tool storage, power tool accessories and...
Replies
2
Views
4K
telectrix
telectrix

Search Forum

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
Top