# 120-240V device on bi-phase service?



## derekleffew (Mar 7, 2012)

So I'm looking at this device:


Doug Fleenor Design - DMX1REL20A
It's just a switch--it doesn't care what you put into it, it will turn whatever goes in ON or OFF.

A portion of the specs:

> Power input: 120 - 240 volts, 50/60 hertz, 2 watts (plus load).
> Power input plug: NEMA type 5-15P dead front plug standard, *others optional*.
> Power output: Input power is switched to output. *(Hot side only)*
> Power output socket: NEMA type 5-15R dead front receptacle standard, *others optional*.
> Output rating: 15 amps with standard connectors, 20 amps with optional connectors.


*BOLD* added by me.

I've used it before (though I can't remember for what [I've also rented one with NEMA 5-15 s for control of a hazer]), and it came from the rental house with L6-20 I/O connectors. Correctly, as we were using it on a 208V line. I didn't realize until just now that it was only switching ONE of the two hot legs. *Is this, or could this, be a problem?*

A related scenario: 
When using any auto-ranging 90-250V device, the power switch probably only interrupts the hot wire. So when connected to bi-phase, the "neutral" is ALWAYS 120V potential difference to ground. *Cause for concern? Should power switches on ALL auto-ranging devices also switch the neutral, just in case?
*


(Standard QotD rules apply: Students ONLY, for one week from date of posting.)


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## sww1235 (Mar 8, 2012)

I would think that switching only one of two hot legs would be dangerous, because there is 120V potential to ground with either 208V or 240V line. I would say that if a power switch is switching only the hot leg on 120V single phase service, that is all it should be used for. If there is a potential for multi voltage use, than both conductors should be switched and fused. 

On a side note, most industrial switches, will switch all wires except ground, so this has precedent

Also how are the control electronics in the DMX relay powered? Does it need a neutral connection? Does it have an auto-ranging power supply?


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## epimetheus (Mar 8, 2012)

sww1235 said:


> On a side note, most industrial switches, will switch all wires except ground, so this has precedent



No, this is a large generalization and not accurate. Many 3 phase disconnect switches and circuit breakers do not switch the neutral, only the phases. Sure, you can wire the 2 phases and the neutral of a 120/240V single phase service to a 3 pole disconnect or breaker, but this is not a code requirement except in certain situations.


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## sww1235 (Mar 9, 2012)

epimetheus said:


> No, this is a large generalization and not accurate. Many 3 phase disconnect switches and circuit breakers do not switch the neutral, only the phases. Sure, you can wire the 2 phases and the neutral of a 120/240V single phase service to a 3 pole disconnect or breaker, but this is not a code requirement except in certain situations.




Sorry, I should not have generalized. I meant that it was not against code


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