such a load of tosh, Ray.I don't know why you are so hard on the guy,he's only asking for advise.design design design.I recon the guy is refering to a 32amp circuit when he talks about 13amp ring main. Its not the rcd bit thats tripping on his rcbo...the type D is to allow for the increased inrush current.Am I missing something here??Whats wrong with using a stand alone rcd at the origin as opposed to rcbo.And type Ds are available in other makes. Yes if using an mcb the design Zs is probably(cant be arsed getting bgb out)around .30 but with the inclusion of the rcd surely that becomes the overiding device when acceptable tripping time is the concern.But having said all that you will probably find that a type c rcbo will cure your problem.I would be more concerned with the different design frequency of your american equipment
Purposely altering a circuit in anyway and finding its Zs thus doesn't comply because of what you have done is a big NO NO! regardless of the fact that a RCD covers the circuit and would seemingly mean it meets BS7671. Following BS7671 when you alter the circuit means your Zs will already meet requirements the clause of using a RCD protection where Zs value cannot be achieved is to achieve a solution with an existing circuit installed possibly under a older version of the reg's for example.
To tell someone to that Zs aint an issue because the RCD will be the overriding device (because this what you imply!) is IMHO very poor advice.
If the OP was testing the install and Zs wasn't met on a circuit he could after ensuring all other aspects of the circuit like volts drop to its use suggest a cheap solution to comply would be to give RCD protection. As the OP is considering altering the circuit he must ensure Zs is met as he becomes responsible for the existing design and his alterations.
The only time you may not be able to achieve Zs through no fault of your own is when the incoming supply is already too high like with a TT and rod..otherwise you must meet it within your design.
I would suggest the OP is getting a little bit of a rough ride because asking such a question as 'can't i just swap a RCBO with a mcb (D)' means he has little or no idea about Earth leakage protection requirements of the circuit cabling or Zs theory and compliance and coupled with this altering a circuit he would be subject to testing it afterwards .... as he seems to have large holes in his knowledge (not small ones where we are always happy to guide) ... i put it that IMO the OP is not competent to do the task with the knowledge he has and gives the impression he won't be routinely testing his work as this would give him the insight as to the concept of Zs requirements and would have been part of his question as oppose to the manor of which he asked his question.
Regardless of the origin of the TX they all can be associated with inrush tripping,what we need to know from the OP:-
-KVA rating of the TX? (this will establish whether suggesting swapping to a type D would even be an option for a ring)
-Whether the existing circuit has compliant test results ?.... (poor insulation value could cause a leakage spike on the inrush of a Tx thus tripping RCD)
-What other equipment shares the same circuit (would nuisance tripping be a big inconvenience 'computers etc')?
A lot of advice is been thrown at the OP without establishing any background details and giving of the cuff advice without the questioning may not cure/resolve his issue.
The suggestion of a separate circuit may be the best option as at present we have so little to go on its as it stands and the OP isn't exactly forthcoming with replies to these questions of which some i have already asked once so until we get a response a separate circuit is the safe advice anyone can give assuming he can design a compliant install.
about design and upgrades ... it may be the case the regs are a little more relaxed domestically surrounded this issue but until im shown good evidence to back up i will assume the above as i have explained is correct ... i am used to installations many orders larger than domestic as you can guess but at the root of it all the maths and theory are the same.... Just ive always been taught that in a TNCS there should be little reason Zs isn't met and most of the time in TNS systems, usually supply earth issues are to blame here though.
