110v transformers and overload on secondary

[Jako13!]

Just trying to try get my head round these 110v wall mounted transformer I have come across. How comes the second side is unfused? Is there any regs on this?

I know the primary usually has has a thermal cut out (is this just purely so we cannot overload and draw too much from the transformer?)

thanks guys for you advice in advance

 

[telectrix]

mytake is that an overload on the secondary will cause a corresponding overload on the primary, thus activating whatever overload device is fitted to the primary. that means that the whole transformer is isolated.

 

[Lucien Nunes]

The ratio of primary to secondary current is fixed, therefore overload protection in the primary can reliably protect the secondary under many conditions and has the advantage of sensing and disconnecting any abnormal primary current that might occur if the transformer developed shorted turns. Obviously if there are outlets on the secondary side that need to be fused down below the transformer rating, then protection is needed there too.

In the general case of a transformer with centre-tapped secondary or multiple windings protected only in the primary, it might be possible to overload one part of the secondary even while the total VA load and hence primary current remain within normal limits. In the case of the tool transformer, the CT is never used by the load i.e. it does not serve as a neutral, therefore it is reasonable to provide overload protection only for the full winding, so long as the two halves have fault protection to deal with shorts to the CT. This is easy to achieve via the primary since the fault current is much higher even when only half the winding is involved.

 

[pc1966]

As always Lucian's description is quite comprehensive. For large site transformers (i.e. the 55-0-55V style of 110V with CT) it is also common to have double-pole MCBs for protection of each of the lower current outlets to deal with the risk of a line-earth short not being protected in any meaningful way by the primary OCPD.

For example, a 16A MCB on the primary of a 230-110V transformer is equivalent to 33.5A on the 110V secondary, but double that at 67A for a 55V line-earth (CT) short!

 

[Jako13!]

As always Lucian's description is quite comprehensive. For large site transformers (i.e. the 55-0-55V style of 110V with CT) it is also common to have double-pole MCBs for protection of each of the lower current outlets to deal with the risk of a line-earth short not being protected in any meaningful way by the primary OCPD.

For example, a 16A MCB on the primary of a 230-110V transformer is equivalent to 33.5A on the 110V secondary, but double that at 67A for a 55V line-earth (CT) short!

Makes sense so the primary wouldn’t detect that hence using smaller breakers on the secondary to allow for it. So in theory on the smaller types the internal overload protection on the primary should allow for the worst case to protect if a earth fault occurred or surly we’d have a very hot transformer and melted cables?

 

[pc1966]

Makes sense so the primary wouldn’t detect that hence using smaller breakers on the secondary to allow for it. So in theory on the smaller types the internal overload protection on the primary should allow for the worst case to protect if a earth fault occurred or surly we’d have a very hot transformer and melted cables?

Yes, you can design the transformer to survive a line-CT fault just on the primary OCPD, but for larger ones you generally have essentially a DB on the secondary to provide better and (largely) independent protection on a final-circuit basis.

 

[EricMark]

I have questioned many times the risk of using a 110 volt brick where the overload is on the primary. As said 10 amp at 230 volt is nearly 42 amp at 55 volt, and 1.5 mm² Arctic cable will not take 40 amp without melting so presents a real fire risk.

But wall and floor mounted I have used always had twin pole MCB's.

I have had a burn out where a replacement control 110 volt transformer had an internal centre tap, originally was TT, the neutral wire has an earth fault, and had to do a full rewire of the machine as the burnt wire touched so many others.