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

Yes—those yellow 110 V “site” transformers are legal when they’re 55-0-55 V centre-tapped to earth (RLV) and built to BS EN 61558-2-23, then used per BS 7671 Section 704. The shock risk is reduced (max ~55 V to earth), so RCDs on the secondary aren’t mandatory for RLV sockets; many portable units therefore have only primary protection (thermal fuse/MCB). IET Electrical ExcellenceBSI Knowledgedms.niceic.comIET EngX

What about extra protection?

• Secondary overcurrent: Not always built-in. It’s acceptable to protect on the primary and rely on the transformer’s limited VA if the downstream sockets/cables are adequately rated—but if you’re running multiple outlets/circuits, a small DB with double-pole MCBs/RCBOs on the secondary is good practice (and often used in “site distribution” boxes). IET EngXblakley.co.uk

• RCDs: Permitted but not required on 55-0-55 V systems; adding them is fine (especially for long runs/outdoors), just not a baseline requirement under 704. IET EngX+1

Bottom line:
For two 16 A outlets on a portable RLV transformer, primary protection alone can be compliant; for broader use or mixed circuits, prefer a site transformer with integrated secondary MCB/RCBO distribution for discrimination and convenience. Professional Electrician

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:

The centre tapping only really helps protect against line to earth faults.

Yes ... that's the "protection against electric shock" bit, the theory with the reduced low voltage (RLV) single-phase transformer system being that you can only get a 55 V shock ... and the primary side overcurrent protective device can operate to achieve the 5 s disconnection time required by Regulation 411.8.3 of BS 7671.

With respect to "overcurrent" ... a combination of the primary side overucrrent protective device, and thermal trip, provide for that.

As an example of how ADS might work with a 20 A circuit-breaker on the primary.

A B20 requires Zs to be no greater than 2.19 Ohms at 230 V, which is equivalent to 9.5 mΩ p.u. (per unit). If this is reflected to the secondary at 55 V to Earth, this give 55×9.5÷1000 Ω = 0.52 Ω (which is the value in Table 41.6).

Zs for a 13 A fuse is a similar value of just over 2 Ohms at 230 V.

And, because the primary is supplied line to neutral, you will actually have an effective Zs of even less, assuming you're plugging directly into a socket-outlet in the installation. It also means it's usually OK to plug the RLV transformer into a TT system or IT system ...

To meet voltage drop at an outlet for a supply circuit operating at 20 A, so if my "finger in the air" mathematics is working properly, this is roughly equivalent to 2.5 mΩ p.u. maximum, meaning you are likely to meet disconnection times for 13 a fuse or B20 breaker, even if you use extension leads. Assuming a line to neutral supply impedance of 0.35 ohms (don't forget this is line to neutral not line to earth) this is equivalent to 1.5 mΩ p.u., so in total this gives an overhead of 5.5 mΩ p.u.for the transformer internal impedance line to Earth (not line to line), plus R1+R2 of any extension leads used (this totals to about 0.3 Ohms at 55 V).

(PS - above very quick "finger in the air" or "back of a packet" calculations, no guarantees on accuracy, and apologies if I've missed some assumptions. Please check assumptions and working yourself if reading this e-mail.)