Single Phase Motor Fusing

[gbradley]

Also if the L2 is Fused and the Fuse gets blown, someone might put leads between L1 and a white #2 wire somewhere in the panel.
This person may incorectly assume that there is no power on while L1 is still hot!!!
I Check it to ground -||| also.
I don't work in panels that often, so I have to be more careful than the average Sparky.

 

[TRACY H]

NO SE

I don`t understand WHY we keep calling in a case where the
transfomer only has L1 and L2 120 vac potential. one of them a NEUTRAL

I find it an interesting subject the difference between fusing and not fusing the Neutral depending on if it is a floating circuit or not. The NEC 240.22 would not apply here because it specifically states that a grounded conductor should not be fused. On a floating circuit the Neutral would not be considered a grounded conductor.

I believe WE SHOULD DO AWAY WITH THE TERM NUETRAL If there is such a thing as a NEUTRAL it carries the unbalance between phases as stated in the post above. If you only have L1 and L2 and no center tap on the transformer where is the unbalance going to come from for there to be a NEUTRAL. All we can have is a grounded conductor, which can be L1 or L2 banghead

 

[CaseyK]

ELEVMIKE- Opening One Wire on 220

Mike:

I find HVAC story very interesting. During my stay as an engineering contractor at a (very very) large manufacturer of relays, starters, controls, (plc's, washers, dryers, my new mouse, etc) I ran across the assembly line that made two pole contactors for all kinds of stuph, including air conditioners. Other than being cheap, they didn't look very rugged.

Since I have hooked more residential and small business air coinditioners during the last 5-6 years that I care to remember or admit to, I have seen a bunch of those cheap two pole dedicated purpose contactors. About 2-3 years back, I noticed that they had become One pole, with a solid bus for the second pole. I notice a lot of new air conditioners being installed, with NO disconnect present. Poor service man has his hand in the AC unit for service, with live wires in there.

I was asked about one a little while back, where the only way to kill power to the unit was to shut off the main breaker. Not sure how that one is wired, but some breaker panels have two bolts on the bus bars, I guess just loosen and wrap a wire underneath???

regards.....casey

 

[Eric Nelson]

After or during discovery she got a set of electrical drawings and I noticed that the single phase 220 blower motors were controled by a single contact on the HOT side of the motor. My thought was that the other side was not on a nutural as intended. Turns out she hired another contractor to take a look. He opened up one of the units and found the motor turning slowly with the PB relay open. All the motors where then placed on TWO pole relays....Walla!!! No more burned up motors... (the nutrual was also fixed).

What neutral? With 220V single-phase, neither conductor is considered a neutral (in the US). Yes, a 2-pole relay should have been used, but I don't see where a neutral comes into play here...

beerchug

-Eric

 

[Eric Nelson]

Re: NO SE

If you only have L1 and L2 and no center tap on the transformer where is the unbalance going to come from for there to be a NEUTRAL. All we can have is a grounded conductor, which can be L1 or L2 banghead

"Grounded Conductor", AKA "Neutral". I like the term neutral, because "grounded" and "grounding" are often used interchangeably, which is simply WRONG.

beerchug

-Eric

 

[elevmike]

Eric..

I confused myself and you. It was 10 years ago and I got the story screwed up some with the church organ blower..(different story..long before) which ran on 120 vac.

The probem with the HVAC units was that an open frame motor was used on a rooftop unit. When the motor and associated controls became damp there was a partial ground that caused current to pass through the motor to L2. The circuit drawings looked as if they were intended for a 120 volt circuit that intended to have a nutural on the other side of the motor; which wasnt there because it was a 220 volt motor w/no nutural. So to correct myself..nobody fixed the "nut-rail".

Sorry about that...

 

[Eric Nelson]

The circuit drawings looked as if they were intended for a 120 volt circuit that intended to have a nutural on the other side of the motor; which wasnt there because it was a 220 volt motor w/no nutural.

That makes much more sense. Thanks for the clarification... :nodi:

beerchug

-Eric

 

[rsdoran]

It depends on whose definition you use but in the US its easier to follow NEC standards. Using those stanards if you take an incoming supply voltage with a neutral...ie 120vac from a 240vac split phase panel then its DEFINITELY a neutral and is not fused but may be switched.

If there is a "fault" between neutral and the 120vac I would hope the breaker tripped or fuse blew...this is not a ground fault. This is a shorted circuit. This would also apply to the circuit if both wires were 120vac...ie 240vac circuit. If the 2 phases connect then its a "short".

Ground is or should be at ZERO potential so any voltage "sensed" on ground should trip a ground fault or GFCI. It may not trip a breaker or blow a fuse...just depends. Thats one reason GFCI's are required in certain situations.

Fuses (and/or circuit breakers) are there to protect the integrity of the system. If an overload condition occurs then heat is generated which can cause a fire or other damage, the idea is for the fuse etc to go before damage may occur.

Fuses nor circuit breakers alone can protect against a ground fault nor single phasing. In the case of a ground fault an overcurrent condition may not apply.

In the case where a separate transformer is used and one leg is grounded then that leg is called a "neutral" but by NEC standards it is a grounded conductor. Following NEC standards the grounded conductor should not be fused but again may be switched.

I have the vocabulary (and hopefully some knowledge) but not the words to properly state the information.

As for the original question:

I am curious as to how many of you fuse L1 and L2 before a motor starter to a single phase 120VAC motor. We have some people here that have said that it is required, but I don't see why you could not just fuse L1.

If the supply voltage is 120vac and has a neutral or grounded conductor then just one leg is fused.

We had a similar discussion on something similar here:
http://www.patchn.com/forum1/viewtopic.php?t=476