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Fuse size and type for 230V / 110V transformer

Tom D

Can someone confirm if in place of FUSE 1 ca be used 25A RCBO Type D and in place of FUSE 2 16A MCB Type C  please?

Wall mounted transformer is 3.3kVA 230V/110V as per the below snapshot. 

Will 4mm2 singles H07 be fine on the primary and secondary side of the transformer if we don't consider any major voltage drop?

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    Presuming it's a centre-tapped RLV arrangement, the secondary MCBs would need to be full double-pole (not single pole, nor switched N), and perhaps need RCD backup if the loop impedances weren't low enough to give 5s disconnection times (an RCD on the primary wouldn't see earth faults on the secondary side). C16 sounds about right.

    A D25 on the primary is harder to judge - it all depends on the starting surge of the transformer (and any connected loads, if they might be present on power-up) - I'd say it's in the right ball park, but verify against the manufacturer's instructions and likely usage conditions. Probably a simple MCB will do rather than an RCBO unless there are extenuating circumstances (like it's in a bathroom or a TT system).

       - Andy.

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  • 0

    Presuming it's a centre-tapped RLV arrangement, the secondary MCBs would need to be full double-pole (not single pole, nor switched N), and perhaps need RCD backup if the loop impedances weren't low enough to give 5s disconnection times (an RCD on the primary wouldn't see earth faults on the secondary side). C16 sounds about right.

    A D25 on the primary is harder to judge - it all depends on the starting surge of the transformer (and any connected loads, if they might be present on power-up) - I'd say it's in the right ball park, but verify against the manufacturer's instructions and likely usage conditions. Probably a simple MCB will do rather than an RCBO unless there are extenuating circumstances (like it's in a bathroom or a TT system).

       - Andy.

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