Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Replies 13 replies
  • Subscribers 39 subscribers
  • Views 1700 views
  • Users 0 members are here
Options
You may also be interested in

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

  • 0 in reply to KC

    what disconnect time ? - its 55V to earth before its pulled down by the fault current, and probably way less than 50 at the fault, more like 25-30V 

    Mike.

  • 0 in reply to mapj1

    what disconnect time ?

    BS 7671 still asks for a 5s disconnection time for earth faults for RLV systems - 411.8.3 (despite the underlying physics/biology not really requiring it).

    I was thinking about long cable runs on the secondary,

    If cables are too long you're likely to hit voltage drop problems first (although admittedly those may be overlooked in practice). As a rough rule of thumb, if voltage drop requirements are met (and c.p.c.s are full size) ADS is likely to work for C-type MCBs - B-types should therefore have no problems and fuses should be OK (certainly for 5s disconnection times).

       - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    BS 7671 still asks for a 5s disconnection time for earth faults for RLV systems - 411.8.3 (despite the underlying physics/biology not really requiring it).

    BS 7671 permits 5 s on distribution circuits, and heavy-current circuits, in TN systems (and 1 s for the same classifications of circuits in TT installations). 

    The physics/biology doesn't support this either for all possible touch-voltage scenarios, although inside what we used to call the "equipotential zone", in TT systems, touch-voltages should be relatively low, and in TN systems in the "equipotential zone" touch-voltages are likely to be relatively low.

    5 s (and 1 s) is really a "pragmatic" time.

Community Rules & Guidelines