Controls in the grounded side

[ronaldrc]

Just wanted to show the reason not to put the start stop buttons in the grounded side of a magnetic starter if you don't need to.


http://home.comcast.net/~ronaldrc/wsb/html/view.cgi-image.html--SiteID-875254.html

Ronald

[ August 22, 2003, 10:02 PM: Message edited by: ronaldrc ]

 

[pwhite]

Re: Controls in the grounded side

ronald,

nfpa 79 - electrical standard for industrial machinery gives examples of the proper wiring methods. clearly not to their standards.

 

[ryan_618]

Re: Controls in the grounded side

Would 404.2 (B) not apply?

 

[bennie]

Re: Controls in the grounded side

The schematic illustrates why the control transformer is an isolation transformer. The secondary should not be grounded.

 

[ronaldrc]

Re: Controls in the grounded side

Hi Bennie good to see you back.

This ruling applies only to a grounded secondary.

I think this is a good idea why would you want to put the start stop in the grounded side anyway?

As far as I know you can do any control action with magnetic controls in the ungrounded side just as good as you can the grounded side.

The code says this applies only if you leave the starter to a remote start stop, I don't agree with that I think it should apply even if you don't leave the starter enclosure.

I don't think many electricians would put the start stop in the grounded side on purpose.

I know in the past I have seen that the control transformer wasn't grounded and in a hurry or not tracing have grounded the side that had the start stop by mistake.

No one ever ask me if I grounded the right side but if they had at that time I would have said it wouldn't make any difference but now I know better.

Ronald

[ August 25, 2003, 03:12 PM: Message edited by: ronaldrc ]

 

[bennie]

Re: Controls in the grounded side

Ronald: Manufacturers of motor starters install the control wiring to be adaptable to all control voltages. One side can not be grounded.

Reverse the transformer and put in a 240 volt coil. Are you going to ground one end of the 240 winding?

Usually the only change, to make a starter function on 120, 240, or 480 control voltage is to install the proper coil.

 

[don_resqcapt19]

Re: Controls in the grounded side

Bennie,
Every motor starter that I have ever installed that had 120 volt control and a transformer had one side of the control tranformer grounded.

PS: Great to see you back at the forum!!!
Don

[ August 25, 2003, 05:19 PM: Message edited by: don_resqcapt19 ]

 

[ronaldrc]

Re: Controls in the grounded side

Bennie

I believe there refering to motor control centers and control panels with multiple starters in them.

I give you credit for being one of the sharpest individuals on the forums.

You could give me enough credit of knowing I have enough knowledge to know that you can't ground one side of a 240 volt single phase circuit.

Thanks:Ronald

 

[bennie]

Re: Controls in the grounded side

Ronald: I give you credit as a very intelligent individual. I didn't intend to insinuate otherwise. I was only making an anologous comparison.

 

[ronaldrc]

Re: Controls in the grounded side

Bennie

Most of the small motor starters I have installed size 00 to 3 do have line voltage coils as you say. And do not require a transformer.

But if the coil voltage is 120, 24 Ect. as you we need one.

I don't know the va rating off the top of my head but if it exceeds a certain va rating the NEC requires you to ground one side of the secondary.

Ronald

 

[Ed MacLaren]

Re: Controls in the grounded side

you can't ground one side of a 240 volt single phase circuit.

Why not, if it is the secondary of a transformer?

the control transformer is an isolation transformer. The secondary should not be grounded.

It is my understanding that most control transformer secondaries are required to be grounded.
The exception requires that -
a. the primary is not over 1000V,
b. there is qualified maintenance and supervision,
c. a requirement for control power continuity exists,
d. ground detectors are installed on the control system.

In fact, if the control transformer exceeds 1000 va, then the grounding conductor would have to run to a fully qualified grounding electrode, instead of just to the control enclosure.

Or have things changed recently?

Ed

[ August 25, 2003, 08:54 PM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: Controls in the grounded side

No ED, things are the same as you indicate for premises wiring systems, supplied by a power transformer.

My reference is to a control transformer for a motor starter.

 

[ronaldrc]

Re: Controls in the grounded side

Now Eds giving me a hard time I knew when I said that , That someone would come back with that.

Yes Ed we know if it where a 480 system and a transformer was used to reduce it to 240 you could ground one side.

I was refering to direct line to ground without a transformer.

This thing is really getting sideways.

But appreciate the input anyway.

By the way thanks for answering my VA requirement for the grounding of a control transformer I was going to look it up tomorrow.


Thanks:Ronald

[ August 25, 2003, 11:41 PM: Message edited by: ronaldrc ]

 

[roger willis]

Re: Controls in the grounded side

I always thought that even with a control transformer if the primary side was 480 volts we were required to ground one side of the secondary. I know you always see a green ground jumper on the secondary of control transformes in Motor Control Centers.
Is my thinking wrong?

Roger

 

[ronaldrc]

Re: Controls in the grounded side

Roger

I agree with you all motor control centers I have worked on and most control panels with multi motor starters have the secondary grounded.

But like ED said its not required untill the va exceeds 1000 va.

But Ed? Here I go making a mountain out of a mole hill as usual and getting off track.

But would it be practical to have a 240 v. coil in a 480/277 volt motor starter system? Don't mean nothing by the question but the only reason I could see if you had a lot of 240 equipment assocatied with it which is not very likely.


Ronald

 

[brian john]

Re: Controls in the grounded side

You could give me enough credit of knowing I have enough knowledge to know that you can't ground one side of a 240 volt single phase circuit.

Ronald: You could ground the secondary of the 240 vac transformer, most folks wouldn't.

Older MCC's had one single transformer for all the starters with an ungrounded secondary, then individual starters had small 120 vac ungrounded transformers. Now it seems to be the standard (and might me a standard?) that the secondary of 120 vac control transformers are grounded.

 

[bennie]

Re: Controls in the grounded side

The grounding of the secondary is for winding to winding faults.

Next will come a ground for a door bell transformer, an internal fault could apply 120 volts on the push button. Good way to get rid of salesmen.

 

[ronaldrc]

Re: Controls in the grounded side

Bennie :I know you are trying to give me a hard time and you no this.

But it wouldn't matter if the secondary was grounded or not,if you have a fault between the primary and secondary their is going to be a shock hazard anyway to ground.

Ronald :roll:

 

[ronaldrc]

Re: Controls in the grounded side

John I agree with you about the 240 transformer that would be a hex of a shock 240 hurts a lot worse than 120v.

I can't really see why anyone would use a 240 control system on a 480 system.

Ronald

 

[bennie]

Re: Controls in the grounded side

No Ronald, I am trying to remember the design considerations of a control circuit.

A grounded control circuit must be connected to prevent a motor starting, or failing to stop, on a fault condition.

The logic switching will have to be on the L-1 side.

An ungrounded control circuit, the logic can be on either side of the operating coil.

On long runs of control wires, 480 volts can appear as a closed circuit through capacitance coupling. This effect will not cause the motor to start, but can prevent it from stopping.

By lowering the voltage, the decrease in coupling corrects this situation.