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That is true mike and we do it all the time (isolated secondaries). Look at the transformer for a pool light where safety is job #1. It is isolated from ground.

It comes down to what that transformer is being used for. On a service bench, we'll use a 1:1 isolation xfmr with the secondary floating simply to remove the ground reference from the radio/TV being worked on. It reduces risk of shock due to accidental contact and permits grounded test equipment to be connected to "live chassis" apparatus for testing and alignment.

Look at the British bathroom shaver outlet for a similar 1:1 xfmr example. The secondary is left deliberately ungrounded to reduce possible shock risk. They key here is that the xfmr is right at the outlet itself, thus resulting in a short and easily controllable section of ungrounded circuit, just as with test bench example.

If you use those ungrounded secondaries to feed into much more extensive building wiring, capacitive effects at 50/60Hz will start to become much more apparent. Grab hold of just one side of a U.K. shaver outlet and you won't feel much (if anything). Do that with a wiring system running over a much larger area and the stray capacitance could be enough to deliver a significant punch.

Actually no.

I would almost never recommend it for a control transformer, but I have very few reservations about doing it with a large power transfromer. The control transfromer will probably have lousy voltage regulation. But I always check with the transformer manufacturer first.

JBD, you are sounding as if you are on another topic, to me anyway.

The code allows little in regards to isolation transformers, they shall not have grounded secondaries.

You could ground a control xfmr but there are many applications were it would be better to not do so.

Your postion here is not consistent with standard prevention of hazards

517.160 Isolated Power Systems

Also in regards to control xfmr's, there are a boat load of reasons to NOT ground control xfmr's in obvious industrial applications.


Ive wired operating rooms were "isolation" from ground by secondaries is a "shall" issue by all equipment and installation and code.

Furthermore you stated that you must ground all xfmr;s as per 250. I dont agree with that as spec'd above

There is a place in the electrical contruction idustry and elsewhere, ARTICLE 517 Health Care Facilities for not grounding xfrm's secondaries.

This OP was about back feeding a xfmr and evolved to grounding line on the secondary of said back fed xfrm.

JBD, can you elaborate on why you have no problem doing this on large xfmr's and your reasons why NOT to do it on control and isolation xfmr's as the NEC clearly requires.

In my tours of duty in industrial maintenance we requently "floated" the neutral on control xfmr's. It was backed up by the "engineering" staff,. And the NEC.

.

I guess I am, sorry.
I was talking about not reversing connecting control transfromer, I didn't realize we had switched to ground/bond versus float.

Always ground transformer secondaries per 250. I cannot think of any good general reason to float a secondary.

Trumpy,

This happens often for example, a 277V primary and a grounded 120V secondary in a lighting control panel or a 120V primary down to a 24V secondary in a furnce system.

Hi All,
I want to thank everyone for their input on this topic.
One last question:
Does it break any US electrical wiring codes to use a transformer wrong way round, i.e. line connected to primary?

TiA,
Michael.

The line is always connected to the primary. In general the primary is the high voltage winding, but there are no Code issues with treating the low voltage winding as the primary.

You'll get crummy voltage regulation and various other potential issues, but no Code problems.

The distribution system uses step up transformers at the source end so they must be able to handle the voltage regulations problems.

The voltage regulation issue only occurs when a transformer is reverse connected.