Autotransformer supply 4.14.4

[JamieP]

I've been asked to supply a 415V to 380V autotransformer which then supplies a machine thats name plate indicates it is a 380V machine

I've read 4.14.4 a few times and believe that because 415V is the highest input or output voltage that the machine and this means supplying equipment rated at 380V is non-compliant

Is this due to no isolation between windings and that there is potential for 415V across the secondary side in a fault?

Am I reading this right and on the right track? Any input appreciated

[AlecK]

Yes, that's what the clause means.
What you need is a step-down transformer with earthed output

[JamieP]

Thank you Alec

I don't know much about Autotransformers but does anyone know what they would mainly be used for if they can only supply equipment rated at there highest voltage rating? This would make them basically useless for step down applications due to the equipment still needing to be rated for the primary

Only of use in step up applications?

[AlecK]

More that they are too dangerous to be allowed as a form of step-down.
And the greater the step-down; the more dangerous.
Which makes then uncommon; which in turn means most of us are not familiar with them

At one time it was common to find an ELV work lamp tapped off a motor, eg sewing machines.
Cheap to manufacture that way; but a variety of faults can result in 230 V being applied to the ELV side.