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

KC

Hello,

Those portable building site yellow bucket step-down transformers, they don't appear to have any protection on the secondary whatsoever.  There is just a thermal fuse on the Primary.

They commonly have 2 x 16A yellow ceeform outlets, how can this be legal?  

I understand the principals behind centre tapping the secondary of the TX and the safety advantages that offers, but you still have 110v potential between Line and Neutral conductor (or line and line), so surely they should have additional protection?  The centre tapping only really helps protect against line to earth faults.

I see these type of transformers being used for all sorts of applications that require 110v, beyond just building sites, and often wondered is this actually safe?

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?  

thanks

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    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

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  • 0

    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

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