Does the Center Tap Have to be Grounded?
- Thread starter kpepin
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russ
Senior Member
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- Burbank IL
Re: Does the Center Tap Have to be Grounded?
I'm sure you saw 250.21 while you were on that page, right.
[ January 27, 2005, 08:11 PM: Message edited by: russ ]
I'm sure you saw 250.21 while you were on that page, right.
[ January 27, 2005, 08:11 PM: Message edited by: russ ]
kpepin
Senior Member
Re: Does the Center Tap Have to be Grounded?
Secondary of a transformer has X1, X2, X3, X4 with X2 and X3 Connected together and X4 grounded and bonded. This way X1 to X4 will have 240 Volts and X2/X3 to X4 will have 120 Volts. This is common practice for Extruder Controls. It allows you to use a Single pole Breaker for a heater, and single Solid State Relay to control the heater. The transformer ends up pretty balanced because the 120 V is only used for a cabinet light.
Yes, 250.21 covers it. I knew everyone on the forum was pretty sharp and I just wanted to see if anyone else has ever seen this done. It is a great cost saving because you are still breaking all ungrounded conductors and get 240 V from a single pole breaker. That's what economy switching is.
[ January 28, 2005, 06:01 AM: Message edited by: kpepin ]
Secondary of a transformer has X1, X2, X3, X4 with X2 and X3 Connected together and X4 grounded and bonded. This way X1 to X4 will have 240 Volts and X2/X3 to X4 will have 120 Volts. This is common practice for Extruder Controls. It allows you to use a Single pole Breaker for a heater, and single Solid State Relay to control the heater. The transformer ends up pretty balanced because the 120 V is only used for a cabinet light.
Yes, 250.21 covers it. I knew everyone on the forum was pretty sharp and I just wanted to see if anyone else has ever seen this done. It is a great cost saving because you are still breaking all ungrounded conductors and get 240 V from a single pole breaker. That's what economy switching is.
[ January 28, 2005, 06:01 AM: Message edited by: kpepin ]
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: Does the Center Tap Have to be Grounded?
I think if you look closely all ungrounded conductors will need over current protection.
It is very common to have industrial control panels that use ungrounded 120V power. However, if you do this, you must use ground fault detection of some sort. usually this amounts to a couple of pilot lights.
I think if you look closely all ungrounded conductors will need over current protection.
It is very common to have industrial control panels that use ungrounded 120V power. However, if you do this, you must use ground fault detection of some sort. usually this amounts to a couple of pilot lights.
kpepin
Senior Member
Re: Does the Center Tap Have to be Grounded?
All ungrounded conductors do have OCP, but since one leg is grounded, you still only have 1 ungrounded conductor for 240 volts. You also have 1 ungrounded conductor at 120 V. The 2 legs that are 120 V between them and are not grounded just aren't used together or you would need a 2 pole breaker (or 2 fuses) and that defeats the purpose of "Economy Switching"
All ungrounded conductors do have OCP, but since one leg is grounded, you still only have 1 ungrounded conductor for 240 volts. You also have 1 ungrounded conductor at 120 V. The 2 legs that are 120 V between them and are not grounded just aren't used together or you would need a 2 pole breaker (or 2 fuses) and that defeats the purpose of "Economy Switching"
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