Yellow bucket Step-Down transformers, are they actually safe and legal?

KC said:

Should they have an MCB and/or and RCD on the secondary?  They fact they can be powering two different circuits on the secondary surely justifies the addition protection?  

Technically it's one circuit with two outlets, not two circuits (as in BS 7671 circuits are defined by protective devices) - it's common enough to have one circuit feeding multiple socket outlets (especially ones rated 16A or below). Provided there's no specific demand for discrimination so that one socket keeps working if the other supplies a fault, common protection is normally acceptable. Any kind of overcurrent in the secondary (L-N or L-PE) will result directly in a corresponding L-N over-current in the primary (as the energy needs to come from somewhere) - even if it's scaled by the transformer winding ratio (e.g. a 500A fault on the secondary might only result in 250A flowing in the primary if the windings are 2:1), so a device on the primary, if suitably selected, can provide adequate protection for faults on the secondary (and it's often done that way for HV/LV transformers too - to cover faults between before the 1st LV fuse). In that RLV systems don't need prompt disconnection for L-PE faults, even a relatively slow device on the primary (be it a thermal cut-out, the 13A plug fuse or upstream MCB) may well be sufficient.

Shocks L-N (or L-L) generally are a problem and normally neither an MCB or RCD can be relied upon to offer any protection at all. Generally we rely on there being at least two independent layers of insulation and equipment being kept in a reasonable condition. There's no reason why RLV need be any different in that regard.

   - Andy.

KC said:

Should they have an MCB and/or and RCD on the secondary?  They fact they can be powering two different circuits on the secondary surely justifies the addition protection?  

Technically it's one circuit with two outlets, not two circuits (as in BS 7671 circuits are defined by protective devices) - it's common enough to have one circuit feeding multiple socket outlets (especially ones rated 16A or below). Provided there's no specific demand for discrimination so that one socket keeps working if the other supplies a fault, common protection is normally acceptable. Any kind of overcurrent in the secondary (L-N or L-PE) will result directly in a corresponding L-N over-current in the primary (as the energy needs to come from somewhere) - even if it's scaled by the transformer winding ratio (e.g. a 500A fault on the secondary might only result in 250A flowing in the primary if the windings are 2:1), so a device on the primary, if suitably selected, can provide adequate protection for faults on the secondary (and it's often done that way for HV/LV transformers too - to cover faults between before the 1st LV fuse). In that RLV systems don't need prompt disconnection for L-PE faults, even a relatively slow device on the primary (be it a thermal cut-out, the 13A plug fuse or upstream MCB) may well be sufficient.

Shocks L-N (or L-L) generally are a problem and normally neither an MCB or RCD can be relied upon to offer any protection at all. Generally we rely on there being at least two independent layers of insulation and equipment being kept in a reasonable condition. There's no reason why RLV need be any different in that regard.

   - Andy.