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Lets clear this up a bit ......
Rcd's in parallel - general rules.
Putting RCD's in series would give discrimination issues
As the circuit is a distribution circuit reg' 411.3.2.3 allows a disconnection time of 5s
As the dist' circuit is exceeding 32A reg 411.3.2.2 means it dosn't have to trip in 0.4s
Given the above you CAN have 2 rcd's in series if the distribution circuit rcd is time delayed as a time delayed rcd will trip in <5s.
This still wont achieve discrimination though as a N-E fault would still trip both rcd's so to prevent this you can ensure all downstream devices requiring rcd protection after the distribution circuit are protected by a rcd that breaks BOTH live and neutral. reg 531.2.9
So yes you can have 2 in series as above and comply but you need to confirm that the 17th edition board has double pole rcd's as most do if you are fitting a board with rcbos anywhere in it even fitting rcbo's within the unprotected ways of the 17th edition board then you will have issues as these are not double pole and you wont achieve discrimination.
In the design stage just having an swa to start with saves alot of time and can be cost effective believe it or not after all the supplementary earthing with the sub-main or time wasted trying to ensure the earthing in the t&e is sufficient.
Additional edit ....as had breakdown and didnt have time to look it up...
Your set-up with cable <50mm and no mechanical protection!
Now for the technical bit
........ Reg 522.6.7 species that if all points of reg 522.6.6 are not met in this case a t&e less than 50mm below surface without mech' protection then RCD protection is required-- so far straight forward then it refers to the characteristics of the rcd must comply with reg 415.1.1. which specifies that the rcd must not exceed 30mA and an operating time not exceeding 40ms at a residual current x5.
Thus we now refer to appendix 3.... now this is where as pointed out earlier you confuse us... time delay rcd's start from rating 100mA but looking at the table and assuming you can purchase a 30mA rated S-type it shouldn't effect the trip times against other ratings of S-Type, if you look in the X5 column the tripping characteristics clearly show S-types trip between 40 and 150ms which would mean your install would contravene regulation 415.1.1 because you are using rcd to provide additional protection so it must trip within 40ms at x5 which cannot be done with a time delayed rcd so a standard rcd must be fitted and then we are back to square 1 as you dont have discrimination of rcd's.
Solution ...Swa or othe mechanically protected set-up.
Its ok that your old boss was laying down the rules to you but he also needs to learn his job he's used a set of regulations to show compliance but ignored others that have priority on the situe..... hopefully after all my input here we cn lay this to rest that you can't have in your design a time delay rcd covering the sub-main because it is required for additional protection so must be a standard rcd 30mA with no delay.
Rcd's in parallel - general rules.
Putting RCD's in series would give discrimination issues
As the circuit is a distribution circuit reg' 411.3.2.3 allows a disconnection time of 5s
As the dist' circuit is exceeding 32A reg 411.3.2.2 means it dosn't have to trip in 0.4s
Given the above you CAN have 2 rcd's in series if the distribution circuit rcd is time delayed as a time delayed rcd will trip in <5s.
This still wont achieve discrimination though as a N-E fault would still trip both rcd's so to prevent this you can ensure all downstream devices requiring rcd protection after the distribution circuit are protected by a rcd that breaks BOTH live and neutral. reg 531.2.9
So yes you can have 2 in series as above and comply but you need to confirm that the 17th edition board has double pole rcd's as most do if you are fitting a board with rcbos anywhere in it even fitting rcbo's within the unprotected ways of the 17th edition board then you will have issues as these are not double pole and you wont achieve discrimination.
In the design stage just having an swa to start with saves alot of time and can be cost effective believe it or not after all the supplementary earthing with the sub-main or time wasted trying to ensure the earthing in the t&e is sufficient.
Additional edit ....as had breakdown and didnt have time to look it up...
Your set-up with cable <50mm and no mechanical protection!
Now for the technical bit
........ Reg 522.6.7 species that if all points of reg 522.6.6 are not met in this case a t&e less than 50mm below surface without mech' protection then RCD protection is required-- so far straight forward then it refers to the characteristics of the rcd must comply with reg 415.1.1. which specifies that the rcd must not exceed 30mA and an operating time not exceeding 40ms at a residual current x5.Thus we now refer to appendix 3.... now this is where as pointed out earlier you confuse us... time delay rcd's start from rating 100mA but looking at the table and assuming you can purchase a 30mA rated S-type it shouldn't effect the trip times against other ratings of S-Type, if you look in the X5 column the tripping characteristics clearly show S-types trip between 40 and 150ms which would mean your install would contravene regulation 415.1.1 because you are using rcd to provide additional protection so it must trip within 40ms at x5 which cannot be done with a time delayed rcd so a standard rcd must be fitted and then we are back to square 1 as you dont have discrimination of rcd's.
Solution ...Swa or othe mechanically protected set-up.
Its ok that your old boss was laying down the rules to you but he also needs to learn his job he's used a set of regulations to show compliance but ignored others that have priority on the situe..... hopefully after all my input here we cn lay this to rest that you can't have in your design a time delay rcd covering the sub-main because it is required for additional protection so must be a standard rcd 30mA with no delay.
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