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RCD Types for EV Chargers on TT systems

Former Community Member
Former Community Member

On a TT install, where the supply for EV chargers comes from a feeder pillar. If the EV charger has a 30mA Type B RCD, is there a requirement for an upstream 300mA Type B RCD or can it be a different type? 
Can I specify 300mA RCDs as the main switch (is this type B?), and what would the solution be if I wanted a 250A incomer, would I then need an additional upstream type B RCD for each charger between the distribution board and the charging unit. This is a very expensive component if needed.
Siemens and an article from Voltimum have indicated that the upstream RCD needs to Type B and the code of practice shows a 300mA RCD upstream on a diagram but not which type. 
Looking for some guidance on this and an explanation if possible.

Thanks

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    AJJewsbury: 
     

    I thought the problem was that 30mA B-type RCDs might not trip until the d.c. residual current reaches 60mA - whereas 6mA d.c. might be enough to interfere with the operation of A-type RCDs.

    Yes, this is the usual claim when equipment poses a risk of giving smooth dc residual (because of not either having a design to avoid it or having its own detection to limit it to a few mA).  The type-EV RCD has a tighter upper limit on what smooth dc residual it will permit, intended to make it ok for upstream type-A.

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    AJJewsbury: 
     

    I thought the problem was that 30mA B-type RCDs might not trip until the d.c. residual current reaches 60mA - whereas 6mA d.c. might be enough to interfere with the operation of A-type RCDs.

    Yes, this is the usual claim when equipment poses a risk of giving smooth dc residual (because of not either having a design to avoid it or having its own detection to limit it to a few mA).  The type-EV RCD has a tighter upper limit on what smooth dc residual it will permit, intended to make it ok for upstream type-A.

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