Switching Relay Safety Issue

[dabatman]

I have a customer who has wiring in place using a relay to switch a 120V circuit as a means of remote power switching. The new equipment we are installing is using a power supply that has variable voltage input. Now the power supply can take 208V (vs. 120V), so they suggested running 208V power to it and putting the relay on one of the hot legs of the circuit. This would only switch one leg and the other leg would still be hot. It seems to me this is a bad idea, but I can't find anything explicitly in the NEC saying you can't do this.

The power supply would be grounded and technically the circuit would be "switched off" but again seems like this shouldn't be done this way. Trying to convince them we need a relay on each hot phase but I would like some reasoning to back it up other than, "it's a bad idea".

Thanks.

 

[iwire]

In general the NEC allows the control of equipment via just one leg or two if three phase but the disconnecting means for servicing the equipment must open all ungrounded conductors simultaneously.

 

[dicklaxt]

I'm not following this scenario,,,,,,,,,,where is the 120v circuit fed from(I would assume a UPS source) and what remote power specificly is it switching?

Is the new power supply/equipment what you are switching the power on and "remote switching" the terminology you are using to accomplish this function?

dick

 

[dabatman]

The power is being fed from a standard breaker with an inline relay near the power connection (to open the circuit). A control panel is controlling the relays. The new equipment can't fit on 20A relay (the old setup was 120V) without changing the power to 208V and so now the relay is switching one of the hot legs.

The servicing disconnect will be a 2pole breaker feeding the new 208V circuit so as far as servicing goes, the disconnect will be the breaker.

 

[kspifldorf]

That's done very often, most hvac controls are like that, even your water heater is doin it. As long as your disco is breaking both lines, it's ok.

 

[dicklaxt]

Switch both legs of the 208v feed by using two contacts from the same control relay,one in each 208v hot leg.Double check amp rating on relay contacts to see if they can switch the 208v power supply under load,if they are not heavy enough contacts you can switch a power relay or contactor with the control relay if need be so that the 208v can be routed thru the power relay or contactor that does have beefy enough contacts.

dick

 

[iwire]

That's done very often, most hvac controls are like that, even your water heater is doin it. As long as your disco is breaking both lines, it's ok.

I agree.

 

[dicklaxt]

I agree as well,it is often done,,,,,,,, but if there are problems with in the process unit or on the skid itself that dictates a shutdown and I were an electrician working on that skid I sure would be glad to have it open both legs right then and there.

dick

 

[iwire]

I agree as well,it is often done,,,,,,,, but if there are problems with in the process unit or on the skid itself that dictates a shutdown and I were an electrician working on that skid I sure would be glad to have it open both legs right then and there.

As an electrician I would like a lot of things but I don't get them. :grin:

This type of issue is exactly why only qualified people should be servicing the equipment and qualified people know to use the required disconnecting means before assuming the equipment is dead.

 

[dicklaxt]

True enough but the OP'er and his assumed client were concerned with safety and if it is important enough to have a control relay shutdown the power then what ever triggered the controlled shutdown is not in a normal operating mode and needs to kill everything.IMHO:grin:

dick

 

[mcclary's electrical]

I'm not following this scenario,,,,,,,,,,where is the 120v circuit fed from(I would assume a UPS source) and what remote power specificly is it switching?

Is the new power supply/equipment what you are switching the power on and "remote switching" the terminology you are using to accomplish this function?

dick

he's curious about breaking one of two legs of 240 volts, which is perfectly fine for a controller

 

[dabatman]

Thanks everybody. I appreciate the help.