500va Transformer

[bernardgreen]

Protection-wise, if you wanted/needed RCD protection on the secondary side, you would have to have one end of secondary connected to earth and install an RCD on the secondary side,

The protection on a 50-0-50 supply for control and low energy actuators is not normally for protection from electric shock but more for the protection of the equipment during fault conditions.

Some systems have used an RCD with the centre tap of the 50-0-50 bonded to the metal work. Then any single short to metal work that would otherwise go un-noticed will trip the RCD and invoke an inspection / repair of the equipment. A lot of equipment operating on ELV will tolerate a single short to chassis / framework until a second fault occurs.

 

[JohnW2]

Some systems have used an RCD with the centre tap of the 50-0-50 bonded to the metal work. Then any single short to metal work that would otherwise go un-noticed will trip the RCD and invoke an inspection / repair of the equipment. A lot of equipment operating on ELV will tolerate a single short to chassis / framework until a second fault occurs.

That makes some sense - although a single 55V AC supply, let alone a 110V AC supply with earthed centre tap, is actually LV, not ELV, isn't it?

Kind Regards, John

 

[Jackrae]

If you do indeed want to use it as a "control" transformer then, as others have suggested, there is no need to earth any of the secondary poles since there should be no way a user can make contact with any of the system voltage. However, a couple of questions: why do you want to control at 55 volts (obtaining 55volt contactor coils may be a problem) and why do you want to use such a large transformer for such a circuit. Unless of course it just happened to be ready to "fall off the shelf". 110 and 24 are "normal" AC control voltages.

 

[wreck-it-steve]

If you do indeed want to use it as a "control" transformer then, as others have suggested, there is no need to earth any of the secondary poles since there should be no way a user can make contact with any of the system voltage. However, a couple of questions: why do you want to control at 55 volts (obtaining 55volt contactor coils may be a problem) and why do you want to use such a large transformer for such a circuit. Unless of course it just happened to be ready to "fall off the shelf". 110 and 24 are "normal" AC control voltages.

The control voltage is 110v (the transformer is 55v-0v-55v) and the reason its so big is because it was the only one I had left without having to buy another one.

 

[ericmark]

With a 55-0-55 volt supply every MCB needs to be double pole and all switches need to be double pole this is not normal with a control circuit normally they have fuses or single pole MCB's in the line only and the switches only control the line.

I had to completely re-wire a machine where a 55-0-55 transformer was supplied in error with the earth connection internally linked in the transformer. There was a neutral - earth fault which was unfused and the neutral wire burnt through many other wires within the same conduit. This in turn caused 110 and 230 volt to be cross linked and loads of very expensive blue smoke.

So in normal circumstances I would not use a 55-0-55 transformer for control circuits one where it also had to provide power to a 110 socket as well would I even consider it and even then likely I would use two independent transformers.

 

[JohnW2]

With a 55-0-55 volt supply every MCB needs to be double pole and all switches need to be double pole this is not normal with a control circuit ....

The OP almost certainly does not want or need a 55-0-55 supply for his 'control' purposes. He just wants a 110V supply, and therefore can just ignore the centre tap of his transformer (remember, it's just a bare transformer, not some constructed power supply or generator) - the only real question is whether he just leaves the 110V supply floating or whether he connects one side of it to earth.

Kind Regards, John

 

[JohnW2]

... why do you want to control at 55 volts (obtaining 55volt contactor coils may be a problem) ...... 110 and 24 are "normal" AC control voltages.

My understanding (perhaps wrong!) is that the OP wants 110V, not 55V for his control circuits, and simply happens to have a 55-0-55 transformer (which is fine - he can just ignore the centre tap).

Kind Regards, John

 

[ericmark]

The transformer which was supplied in error where I was working had the centre tap connected to the frame of the transformer there was no way to disconnect the centre tap and make it into a 0-110 volt transformer.

The options were buy a new transformer or.
Fit fuses to what was originally the neutral.

Since we had din rail mount fuse holders is stock and it was a full rewire anyway we fitted fuses to line 2 which with original design was neutral.

He may be lucky and he may be able to remove the centre tap and make it into a 0-110 transformer but he also may have the same problem I had. If I had been fitting the transformer I would have returned it as unsuitable but by time I got involved it had been running for a couple of years and no one including me had realised the error until the burn out.

 

[wreck-it-steve]

The transformer which was supplied in error where I was working had the centre tap connected to the frame of the transformer there was no way to disconnect the centre tap and make it into a 0-110 volt transformer.

The options were buy a new transformer or.
Fit fuses to what was originally the neutral.

Since we had din rail mount fuse holders is stock and it was a full rewire anyway we fitted fuses to line 2 which with original design was neutral.

He may be lucky and he may be able to remove the centre tap and make it into a 0-110 transformer but he also may have the same problem I had. If I had been fitting the transformer I would have returned it as unsuitable but by time I got involved it had been running for a couple of years and no one including me had realised the error until the burn out.

I dont unerstand... ALL control panel transformers I can find now are 55-0-55v

 

[Simon35]

I was going by what Jackrae wrote: "The concept of 55-0-55 is for the 0 point to be grounded (earthed) such that neither output terminal of the 110 volt secondary exceeds 55 volts with respect to earth. All UK 110 volt utility tool transformers (the yellow ones) are wired like this (or at least they should be)"

Yes, what Jackrae wrote is true of site tools in the UK (e.g. as used on construction sites), but we have no idea of what sort of equipment you plan to power from this transformer, or how you plan to deal with protective measures.

Kind Regards, John

The transformer will provide a control circuit in a control panel (contactors, relays, lamps etc) the primary side will be protected by a MCB and the secondary either single pole MCB, double pole MCB or quick blow fuses

Not yet sure on the correct protection for the secondary side.

Normal way in a control panel is to tie one 55V secondary leg to panel earth, the other 55V leg provides 110VAC for your control circuit, ignore the centre tap, single pole MCB/fuse for protection.

 

[stillp]

I dont unerstand... ALL control panel transformers I can find now are 55-0-55v

They are not control transformers. You're looking in the wrong place(s).
As has been said, you should not connect the centre point to earth if you're going to use a 55-0-55 transformer as a control transformer. There's a risk that a fault might put half the supply (i.e. 55V) across a relay coil, which might be enough to prevent it releasing when it should. See BS EN 60204-1.

 

[ericmark]

I dont unerstand... ALL control panel transformers I can find now are 55-0-55v

Shown is a panel transformer these are often more expensive than the typical 55-0-55 volt version as designed to run 24/7 at full load and produce far less heat than the 55-0-55 volt version.

Clearly this was why we ended up with the wrong one some one was trying to get on on the cheap.