3 or 4-Pole Automatic Transfer Switch (Transformers as source-No Gen) 2

[guiyermo]

Hello all,

Currently I am looking at a 3-Phase 4-Wire system (208/120 V) which will be supplied from an Automatic Transfer System (ATS). This ATS does not have the typical Normal and Emergency sources one can find in regular ATS systems: step down transformer (Normal) and the generator (Emergency). In this case both sources are two identical delta-wye transformers (100 to 150 kVA) with secondary (208/120 V) solidly grounded. I have spent the last day and a half reading other threads but I still don't find a solid answer.

I would like your opinion on the following question:

Is it required a 4-pole ATS?
Thanks,

 

[rbulsara]

Rules remain the same, regardless of source.

If the neutral of the two transformers are separately grounded, you need a 4-pole switch.

If the both neutral are tied together and the tied to ground at a single point, you can use a 3 pole switch.

If the loads served do not require a neutral, you obviously do not need a 4 pole switch.

Rafiq Bulsara

http://www.srengineersct.com

 

[Thriggen]

I just recently installed a transfer switch. This is just more information to chew on.

http://ecmweb.com/mag/electric_switching_neutral_whats/index.html

http://ecmweb.com/power_quality/electric_groundfault_current_problems/index.html

Hope you can glean something from them.

 

[Kiribanda]

guiyermo,
The two sources are considered as "sepertaly derived" eventhough they are identical.Therefore,you need a 4-pole ATS with an overlapping neutral.Also the neutral has to be fully rated as the phase poles.

 

[davidbeach]

No, 4 pole is not an automatic requirement. See Rafiq's response.

 

[waross]

I agree with Rafiq.
If both of your source transformers may share a common neutral to ground connection, you may then use a three pole switch. I doubt very much that this is the case here.
If each transformer has its own neutral connection to ground, you must switch the neutral. The Canadian code requires all 208/120V systems to be grounded and I believe the NEC has a similar requirement.
The ECM article is concerned with the different types of neutral switching transfer switches and there terminology is somewhat flawed. There are available four pole transfer switches in which all four poles are basically identical but with closer contact spacing and some resilience in the contacts, an overlapping neutral may be provided. One example may be a four pole contactor with thicker contacts on the neutral pole.
I have several times seen the damages caused by an open neutral.
I have also seen some transfer switches of incredibly bad design. Some would not work, right out of the box and about six were replaced within a year due to failures.
I usually go with the best price, BUT NOT FOR TRANSFER SWITCHES. Take a look at the mechanism. Simple is better. Contactors are good and breakers with motor operators installed work well.
With contactors and breakers, simultaneous operation of the neutral is acceptable. Are both transformers from the same source or is one fed from a generator? Transferring between supplies that are always in phase may be an issue. The operating time of the transfer switch may be enough that the back EMF from your motors slips badly out of phase. This may result in severe current spikes and breaker tripping and may lead to motor damage. You don't want a fast transfer. Either a slow transfer or a super fast transfer. There are some switches that transfer so fast that out of phase closures are not an issue.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[guiyermo]

Hello,

Thanks all for your responses. In the system we are dealing with the two transformers are fed from Bus A and Bus B of a 15 kV Switchgear which will normally operate with its tie breaker in open position.

The two transformers are not close to each other so the grounding of the neutrals will be done independently, although the ground system is the same.

The only motor load is an Air Handling unit, the rest of the loads are battery chargers, lights and receptacles.

Based on the all of your contributions I conclude that a 4-pole ATS is required and it is recommended to have an open transfer, allowing enough time for the motor loads to slow down and avoid damage.

Thanks all,

Guiyermo