Fusing Class 2 transformers

[captklutz]

Need some advice here. I install temp control systems and have an engineer that wants the secondary side of the x-former to be fused. reading the code about x-formers, class 2 is such that it is inherently(sp?) protected. I could not find anything about fusing such x-former.
What I have is a x-former, 24 volt. each wire will have 12 volts unless you ground one leg then you have 24v to ground. Our installation practice is not to ground any of the legs and we set on 50VA x-former for the controller and a 96VA x-former to power the controlled devices.

On the 50VA x-former, no legs are to be grounded. On the 96VA we can ground one leg to remove noise that the sensors get from some acuators we use. I will not go into detail on this part.

So I am thinking that by code all ungrounded conductors need to be fused. Thus I feel I would need 3 fuses min. 4 at the most.

Am I correct in my thinking? I have argued my thoughts with my co-workers who insist on just putting one fuse for each transformer grounded or not.

Thanks!

 

[dereckbc]

Re: Fusing Class 2 transformers

I am no expert on class 2, but if there is no code requirement to ground any of the circuit conductors, there is no harm in doing so if required by design specification other than putting an egg in the basket.

I would simply call the spec engineer and ask him what his/her thoughts are.

[ September 28, 2003, 10:13 PM: Message edited by: dereckbc ]

 

[websparky]

Re: Fusing Class 2 transformers

Based on past servicing of machine control circuits;

There are times when fuses are added to the secondary side when the intent is to protect the controls on the secondary circuit. This was done when the output of the xfer was higher than the rating of some of the costly components.

 

[don_resqcapt19]

Re: Fusing Class 2 transformers

Is your transformer labeled as a Class 2 transformer? If it is not labeled as such by the transformer manufacturer, it is not a Class 2 system. If it is not a Class 2 system, it would be a Class 1 Power Limited Circuit. 725.21(A) requires that transformers for Class 1 power limited circuits comply with Article 450. Table 350.3(B) will require overcurrent protection. I see no requirement for overcurrent protection for a Class 2 power circuit. Most, if not all, Class 2 transformers have an internal fuse on the secondary winding. This fuse is not user replacable and is under the outer wrapping of the trasformer windings.
The grounding of Class 1, 2 and 3 circuits is per Article 250. 250.20(A) may require that your circuit be grounded.

 

[captklutz]

Re: Fusing Class 2 transformers

Originally posted by don_resqcapt19:
Is your transformer labeled as a Class 2 transformer? The grounding of Class 1, 2 and 3 circuits is per Article 250. 250.20(A) may require that your circuit be grounded.

Yes, the transformers are class 2 transformers, thus my delema in fusing such an item. All wiring from the system is actually Class 3 cable. very seldom do we have to go with class 1 wiring.

But what I have decided to do is get some Bussman HRJ-LES-fuse fuse holders, one for each leg of the X-former on the 50VA x-former and one on the 96va and ground one leg.

I will check my code book on the article you mentioned. I probably have read it before, because there is some history behind all this that I won't go into at this time.

Thanks!

[ September 29, 2003, 11:14 PM: Message edited by: captklutz ]

 

[jrdsg]

Re: Fusing Class 2 transformers

don's right. all the low-voltage class 2 transformers we install are internally fused.