Transformer

[wellhung]

can someone explain why sometimes the secondary on a control transformer has one side attached to ground and other times it is not attached to ground?

[ThinkingARB]

I usually would not ground power transformers, (those used to provide primary low voltage power to controllers). Transformers used in switching, (relays & triacs) would almost always be grounded to allow hot-side switching. By doing that accidental shorts will not cause field devices to energize.

[wolfdog]

It is a requirement of the NEC.

250.20 (A) Alternating current systems of less than 50 volts SHALL be grounded under any of the following conditions:

(1) When supplied by transformers if the transformer supply system exceeds 150 volts to ground.

(2) When supplied by transformers if the transformer supply system is ungrounded.

(3) overhead conductors

Under these conditions it is basically treated like a seperately derived system and grounding it serves to stabilize the system.

[wellhung]

thinkingARB,

"Transformers used in switching, (relays & triacs) would almost always be grounded to allow hot-side switching. By doing that accidental shorts will not cause field devices to energize"

could you explain this in better detail. thank you.

[heatingman]

They ground one side, and call it common and the other side hot. This is done so to help elimate static electricity shorting on printed circuit boards, as there is less potential difference to cause static. b/c one side of transromer is bonded, and so is the board, then there is no potential difference between the board and the chasis.

[wellhung]

thanks