Type B RCDs

Pl clarify if the switch gear is RCD or RCBO.
RCBO stands for 'Residual Current Breaker with Over-Current', that is RCD + MCB.

Unless you are constrained for space, it is better to put 2 separate items.

It is easier to get spares.

regards,

be careful that type 'B' about a RCBO usually relates to the speed curve of the MCB part, not the DC sensitivity of the RCD part. Check maker's data carefully (if only different committees liaised, we would not use the same letter for two very distinct characteristics.)


It rather depends where the requirement has come from - most designs of VSD rectify the incomong mains to a DC bus. Some designs  may credibly under some odd condition produce a DC fault to earth of the kind that would 'blind' a normal RCD. The question is what happens if the RCD is disabled.

On a TT supply where the RCD is the only form of fault detection, that could be very dangerous. On a TNs supply it may be that the failure of the VSD to the point where a large earth current flows and then a fuse blows instead is acceptable.

The only folk who can really answer that are the maker of the VSD, in terms of the possible failure modes and someone familiar your local installation, as to if RCDs being blinded by DC is an Issue or not.


Personally I suspect you are correct, and the RCD is not always needed, but you need more than a gut feel to back you up before you omit it, especially if it is in the maker's instructions,  as in some cases it will matter.

Mike.

Large d.c. currents can do strange things to RCDs - often making less sensitive rather than defeating them completely - however even less sensitive is probably not what you want if it might affect a 30mA RCD used for additional protection.


EVSE often use a combination of the much cheaper and readily available A-type RCD plus a device that'll trip if the d.c. component of the residual current exceeds 6mA - as A-type RCDs should tolerate 6mA d.c. and remain effective and the problem is disconnected if it exceeds that. That approach can also avoid the need for any upstream RCDs to be B-types too if there are no other loads that can cause a d.c. residual current.


   - Andy.

Thanks for the replies.  The manufacturer of the servo drives recommend a Type B RCD however, the UK distributor usually don't use them in their installs or advise their customers to either. Instead, they recommend additional earth braid bonding to the motor housing.


The supply is I believe TNs.


Although the manufacturer have said to follow the UK distrubutor's advice in other areas where electrical specifications may be intended for other countries, I am erring on the side of caution and using 30mA Type B RCCBs (together with C Curve MCBs) in line with the manufacturer's recommendation.  This usually seems to be the best approach where finger pointing will ensue if an incident were to occur!


Based on test data, typically the servo drive may have a DC leakage of around 10mA.


With two servo drives per RCCB, I'm hoping that the leakage current won't be too high?


Unfortunately, availability of Type B RCDs is difficult at the moment as mentioned above and also due to the global component shortages.


It'd be interesting to know how others address similar requirements at present?

The manufacturer of the servo drives recommend a Type B RCD

Still probably worth clarifying whether the manufacturer means:

a) Should always be supplied via a B-type RCD, or

b) Where RCD protection is required, it should be B-type.


So if b) that might be to your advantage in you have a TN system.


   - Andy.

In a UK industrial environment, it is very difficult to see the use of RCD protection. Additional protection (and you say servo drives) in a bonded environment and one that is maintained does nothing extra, and something like welding robots would probably make short work of RCDs anyway. The question to ask is "why do I need additional protection? What credible problem or accident could there be where it would make any difference". Additional protection is a good idea for portable appliances and places where human direct contact is credible. In a domestic environment, it probably makes a difference, particularly outdoors. In other places, it may increase danger, say my robot loses power to one motion, and it is working near people (some do) then stopping it instantly becomes necessary. RCDs can trip for many non-dangerous situations, and coping with the results is often difficult, say in conveyors in a food factory feeding an oven. A nuisance trip can cost a lot of money in lost product, but it is unlikely to make the factory safer. RCDs everywhere are far from a universal "safety" feature, they may have no credible function in the chain of actual safety.