Supply size

[RF Lighting]

I've just bought a new 10kVA 110V transformer for my workshop, but I can't quite make my mind up what supply to put in for it!

I've got a single phase 50A supply on a type C breaker to the workshop.

I was thinking about putting in a 32A SP supply on a type C to the tranny to allow some discrimination with the submains, and it's highly unlikely the tranny will ever be loaded to capacity anyway, but I'm sure what the inrush current is going to be like, and how well a C32 will cope.

I could put in a 40A supply, but I'm trying to keep costs down, and get away with the smallest supply I can!

Anyone got any opinions?

 

[AdrianUK]

I think you'll be fine on the C32. C type is 5 - 10 times on instantaneous, so that's still 160 - 320A for the tranny switch on surge. And gives you around 63A on the 110V side .... that's a big drill!!! )


The C32 won't discriminate with the C50 on short circuit anyway. I guess my concern would be, on domestic, have you got a high enough PSCC to take out the C50 within 5s?

 

[Adam_151]

He is right, you'll not achieve discrimination anyway, in order not to take the submain on the C50 out, the zs on the final circuit would have to be too high for a B, by which time it'll also be too high for the C32 and your volt drop would be way out of spec as well!

If you think you'll never use it to capacity and are designing on what offcuts of armoured you have in the workshop, then go ahead, 32A or 40A, type C is likely to be fine and you have no discrimination anyway.

I'd be more interested on whats happening on the 110v side, its often thrown together with very little thought I find...

 

[Risteard]

Two BS EN 60898s in series won't give you discrimination under fault conditions regardless of rating.

 

[Lectrician]

Two BS EN 60898s in series won't give you discrimination under fault conditions regardless of rating.

Yesterday, an immersion went squit, took out the 13amp fuse in the FCU/timer, the 16amp MCB, a B40 submain, and a C63 submain!

 

[Spark123]

The biggest problem you'll have is the start up surge, what size fuse does the manufacturer say to use? I remember having to use a 63A fuse on a 415v system for a big 110v transformer, it only fed 16A and 32A sockets (unloaded) but took out 32A fuses for fun.

 

[RF Lighting]

I got the tranny second hand so there's no manufacturers info. There is a C63 on the incommer which is a bit of a worry.

I have a 32A ceeform supply in the workshop on a type C. I think I'll just plug it in there and see what happens!

Here's a couple of pics of the beast. It looks fairly well constructed.

 

[Lectrician]

Have connected a few of these up on sites. Have had to install D types on some. The last one was bunged onto a C32 in existing board, and did trip on initial switch on, but once on, it was never turned off!

 

[RF Lighting]

Best get the MFT out and check my Zs

 

[Risteard]

Two BS EN 60898s in series won't give you discrimination under fault conditions regardless of rating.

Yesterday, an immersion went squit, took out the 13amp fuse in the FCU/timer, the 16amp MCB, a B40 submain, and a C63 submain!

Perfect example!

 

[RF Lighting]

I've been to faults where the local device has not operated, but the cutout has!

 

[Lectrician]

I've been to faults where the local device has not operated, but the cutout has!

I was called out to a house where the chap had replaced a rewireable 30amp fuse with the cu on. Huge bang, cutout fuse blown!

 

[bernardgreen]

An option is to soft start the transformer in a similar way to soft starting a motor.

Or with the output un-loaded switch it on with a zero crossing point semi-conductor switch and then by pass that switch before putting any load on the transformer output

 

[RF Lighting]

Bit of an update. I tried dropping the tranny direct onto my 32A C type supply, and it tripped both the C32 and the C50 submain breakers.

I think this leaves me two options, as I doubt my Ze / Zs will be low enough for D type breakers.

I think I can either look for some sort of soft start device, which I dont know if they even exist!, or look at converting the submains and final circuits to BS88s / BS1361s, which would be a PITA.

 

[SimonH2]

Bit of an experimental suggestion ...
Setup a rig so you can supply it through a small resistance (such as 2 or 3 kW heater), and then short out the resistor.

BTW - I believe that for an inductive load, you don't switch it on at the voltage zero crossing if you want to minimise inrush current.