transfer switch in phase monitor

[StephWri]

I am writing a spec for a transfer switch from an old source I found. That spec specifically prohibited the use of an in phase monitor for the switch. Why would I not want in phase monitoring?

 

[davidbeach]

It depends, maybe you do. Way too many variables and way too little information to even begin to speculate.

 

[rcw retired EE]

Only one US manufacturer had an in-phase monitor a few years ago. Most of the other Auto-Transfer Switches (ATS) operated too slow to use it. The origins of the prohibition may be a suggested spec from a vendor trying to keep his competition out of the bid.

In-phase monitors are useful on transfers from generator back to normal power if the transfer switch has a fast operating time and there are a lot of motors loads.

One type of in-phase monitor looks for the two voltages to be 180 degrees out of phase before it arms the in-phase closing logic. If the transfer switch is transferring between two in-phase utility sources that never get out of phase, the monitor never gets armed to trigger a close signal.

 

[waross]

Not the same as a phase loss monitor or a phase reversal monitor.
respectfully

 

[rbulsara]

If this is an open transition switch, there is no need to have a in-phase monitor. It is only required in closed transition switch, in fact there it is a inherent device in design.

Do not fall for those so called fast actiong ATS with in-phase monitors, they are the most dangerous and useless.

 

[davidbeach]

rbulsara, there are open transition switches that use in-phase monitors. They can produce an open transition so short that computers can ride through, but it is an open transition. Tight in-phase sensing is necessary to avoid problems with motors when having such a short interruption between sources.

 

[rbulsara]

davidbeach:


Exactly what should be avoided.

 

[davidbeach]

rbulsara, my personal preference is closed transition; a few protection issues to deal with but no load problems. I'm not convinced about motors on in-phase monitor transfer switches, but a working in-phase monitor would be a whole lot better than a quick, random phase angle, transfer. A long enough delay to allow all the motors to drop out is certainly the safest. On the other hand, if the load on the generator is mostly lights or (on a optional standby switch) a bunch of computers, the in-phase monitor will keep things running on the transition from generator to utility. Each type has its place.

 

[waross]

Hi davidbeach and rbulsara;
My concern with the fast transfer switches is the very small clearances between the poco feed and the the standby feed contacts. I am concerned that in the event of a forced manual transfer or a transfer with a faulty "In phase monitor", there may be a possibility of an arc-over from the line supply to the standby supply. This would also lead to the possibility of generator damage.
Even without a flashover, there may be a possibility of motor damage.
comments?
respectfully

 

[davidbeach]

waross, as I understand the construction, there is not any less clearance between the fixed contacts than on other styles of transfer switches. The moving contact is larger, and nearly spans the gap during transfer.

A couple of power circuit breakers makes a much better transfer switch.

 

[waross]

Hi davidbeach;
My exposure may be limited.
Most of the transfer switches that I have installed and/or repaired have been manufactured in the UK, and have been magnetic contactors. A few of the bigger ones used motor operators on molded case circuit breakers.
I have only come across three transfer switches with a common movable contact, all of the same brand, out of the US.
They were rated as suitable for manual operation, but the small clearances make my nervous.
I suppose that if the switch did flash over, it would shortly force the generator into sync with the utility and the arc would go out.
respectfully

 

[tommom]

You only need in-phase monitoring under two conditions:
1) closed-transition (ie the generator will momentarily parallel with the utility during re-transfer to energized utility
2) if your system is feeding three-phase motors.

I used to work for Russelectric, and there was a time when they were not excited about in-phase monitors because their motor-operated switches worked very slowly, but then of course, they started offering the in-phase monitor. Before that, they offered a "center-off" position with a variable delay (usually 3 sec.) to allow motor back-emf to decay sufficiently without causing an unduly large current inrush.

Once I repaired a transfer switch in a hospital which had been worked on by the hospital staff. They had reconnected the in-phase monitor leads incorrectly, causing the switch to transfer at 120 degrees - it tripped the utility side circuit breaker every time, until I corrected the wiring.

 

[davidbeach]

tommom, There is at least a third condition where in-phase monitoring is necessary if you are not using a delay in the off position:
3) feeding (certain) 3-phase rectifiers. The filter elements on the front end of the rectifiers, such as UPS systems, can respond adversely to a sudden change in phase angle.

 

[rcw retired EE]

I worked contract maintenance for a major ATS supplier that had fast acting transfer mechanisms and used in-phase monitors when there were motor loads.

I don't recall ever having a flash over in one of their switches except when an electrician dropped a metal piece across the normal supply phase bus on a 4000A switch and took out the hospital. (That was literally on my first day of work. No one was hurt, thank God!)

I made a lot of money, repairing problems and replacing transfer switches composed of two circuit breakers. Sometimes it was a bad design, other times it was poor application or bad maintenance. The basic problems were poor design of motor operators on molded case breakers, poor control circuit design and poor packaging of the switchgear style breakers in cheap switchgear.

Each type has their place. But if you are going to design a transfer system using two breakers, you better know what you are doing.