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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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    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.)

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  • 0
    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.)

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