control power circuits,grounded ,ungrounded

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a.bisnath

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Electrical worker
In my work on many relay control panels with 110 volt control power I have met some where one side of the secondary winding is connected to earth giving a 110 vac to 0 volt potential
I have also met floating secondary windings giving a higher and lower voltage to earth depending on which side of winding is being tested,but across will give the 110vac

Are there any particular reasons or considerations for doing this?
 
An ungrounded system has the advantage of continuing to operate when one of the conductors accidentaly becomes shorted to 'ground'.

A grounded two-wire system only has a 50% chance of doing that.
 
Volta said:
An ungrounded system has the advantage of continuing to operate when one of the conductors accidentaly becomes shorted to 'ground'.

A grounded two-wire system only has a 50% chance of doing that.
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I understand [WWII] naval warships typically floated both sides of power above err ground. The reason was that way, if either side is shorted to ground during a battle, the turret keeps rotating, the guns keep firing. [They actually go one step further; shunting across the fuses with "battleshorts" on the basis it is better to have fires in the wiring than be dead in the water.]

The big issue with floating the control secondary is it can float almost anywhere, and that could be a threat to anyone/anything touching that alleged low voltage. Sometimes one side, or both, is grounded via a resistor sized to pass a few ma. With a balanced scheme, it's a LOT easier to troubleshoot things in the field, by measuring to ground.
 
This typical of substation DC power supply. Typical float voltage to ground is about 66Vdc
 
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