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.