110v fixed wall mounted transformer

[Jako13!]

Hi guys is there any regulations regarding 110v centre tapped transformer transformer circuits being used outside (rcd protected) also if the transformer states 3000Va but a continuous rating of 1650Va am I right in thinking it can only carry 15amps before the transformer overloads?

 

[westward10]

A transformer will often have a continuous and intermittent rating. The continuous rating is a load which can be applied without the transformer suffering detrimental effect in your case 15A. You can exceed this to the transformers maximum load of 27A however not for a continuous period, this would be its intermittent rating.

 

[Lucien Nunes]

EN61558 specifies a 25% duty cycle e.g. 5 minutes on and 15 minutes off for power tool transformer intermittent ratings. Since the copper loss is proportional to the square of the load current, and assuming the duty cycle to be approximately the reciprocal of the heat dissipation when under load, the intermittent rating will be approximately 1/(sqrt (0.25)) = 2 * continuous rating. So a 1.65kVA continuously rated transformer will offer a power tool intermittent (25% duty) rating of around 3.3 kVA. For those 5 minute periods it will deliver 30A and the wiring and OCPD needs to allow for this.

 

[Jako13!]

EN61558 specifies a 25% duty cycle e.g. 5 minutes on and 15 minutes off for power tool transformer intermittent ratings. Since the copper loss is proportional to the square of the load current, and assuming the duty cycle to be approximately the reciprocal of the heat dissipation when under load, the intermittent rating will be approximately 1/(sqrt (0.25)) = 2 * continuous rating. So a 1.65kVA continuously rated transformer will offer a power tool intermittent (25% duty) rating of around 3.3 kVA. For those 5 minute periods it will deliver 30A and the wiring and OCPD needs to allow for this.

What are we relying on for earth fault on this style transformers the primary fuse?

 

[Lucien Nunes]

Very often there will be fuses or MCBs in the secondary too, certainly on the larger transfomers that offer multiple sockets. The kiosk type used on construction sites have what is in effect a complete secondary side DB built in. Secondary protection of CT-E supplies obviously needs to be DP because neither wire of the load circuit is an earthed neutral.

 

[Jako13!]

I was thinking this type? I have seen the ones with a double pole breaker.

 

[Lucien Nunes]

Those, like the yellow portable jobs, usually have a thermal CBE in the primary. I'm not a great fan of the type of CBE normally fitted, a bit flimsy and basic IMHO, but should track the thermal limit of the trafo better than any fuse.