Power

Your questions answered: How to apply transfer switch time duration WCR

Transfer switches are critical to the power system. How do high short-circuit and short-time withstand and closing ratings (WCR) help simplify the selection of overcurrent protection devices? Read more for questions left unanswered during the live webcast

By Hassan Obeid July 8, 2021
Courtesy: Cummins

Transfer switches are critical components in the power system, and they are tested to meet UL 1008: Standard for Safety – Transfer Switch Equipment. This course discussed some of the UL 1008 short-circuit testing and passing criteria and then demonstrated that high short-circuit and short-time withstand and closing ratings (WCR) help simplify the selection of overcurrent protection devices and enable selective coordination strategies. It also touched on the differences between switchboards (UL  891) and switchgear (UL  1558).

Learning Objectives

  • Discuss the UL 1008 short-circuit testing requirements and derive the WCR.
  • Learn about the difference between the short-circuit WCR and the short-time rating.
  • Explain how the time-based short-circuit WCR simplifies the selection of breakers.
  • Examine how the short-time WCR enables selective coordination strategies.
  • Deliberate the major differences between switchboards and switchgear (UL 891 and UL 1558).

On June 29, 2021, the webcast How to Apply Transfer Switch Time Duration WCR left some questions unanswered. They’re addressed here by Hassan Obeid, Global Technical Advisor for Systems and Controls – Critical Protection, Cummins Power Generation.

Q: Can you explain what an any breaker rating is?

A: The any breaker rating is a common way to describe the 3-cycle or 50ms time-based rating. The term “any breaker rating” comes from the UL 489 requirement that molded case circuit breakers rated 400 amps and above must clear a fault in no more than 3 cycles or 50ms.

A breaker that clears a fault in 3 cycles is suitable to protect a transfer switch that can withstand a fault for 3 cycles.

The any breaker rating applies only to breakers that are rated to interrupt available fault current within 50ms. The term “any breaker” is commonly used interchangeably with the 3-cycle or 50ms rating.

Hassan Obeid, Cummins

Hassan Obeid, Cummins.

Q: When would you use a 4-pole switch? How would you handle ground faults?

A: Ground fault sensing depends on being able to sense ground fault current. In order to accurately sense ground fault current, it must return to its source on a known path relative to ground fault current transformers (CTs). In basic emergency standby systems, there are two rules to follow to meet these requirements:

1) There can be only one neutral/ground connection on any neutral bus at one time.

2) Ground fault sensors must be downstream (or on the load side) of the bonding connection.

In order to meet both of these rules when connected to either the normal or the emergency source, the neutral must be switched using a 4-pole transfer switch.

This does get a bit more complicated with paralleled sources, but the basic concept that reliable ground fault sensing requires fault current to return to the source on a known path still applies.

Q: Are any automatic throwover systems listed to UL 1008?

A: Automatic throwover systems are typically listed to UL 891, and they are mostly main-tie-main with a programmable logic controller (PLC). UL 1008 doesn’t allow the inclusion of a tie breaker.

Q: Will service entrance rated transfer switches have a WCR, a short circuit current rating or both?

A: Service entrance rated transfer switches still have a WCR, but since the breaker is part of the transfer switch enclosure and wired upstream from the normal side, it is accepted to have an ampere interrupting capacity (AIC) rating.

Q: What level of testing is needed for the transfer switch once on-site and installed, before turning over to a prospective owner?

A: There are some tests that must be completed during the commissioning process. These tests are outlined by the transfer switch manufacturers in the installation manuals. There are no tests required by UL during this phase.

Q: How do you increase the withstand rating of automatic transfer switch (ATS)?

A: Transfer switches have several different WCRs, depending on how long it takes for the specific overcurrent protection device (OCPD) to clear the fault. The faster the fault is cleared, the higher the WCR.

Q: How many tests are performed? Is it just one instance?

A: Yes, UL 1008 requires one sample to be tested. If there is any change to the material, design, etc., then a new test is required by UL.

Q: Please clarify the difference between the terms switched neutral, solid neutral and overlapping switched neutral. And for which application should each type be used?

A: When applying a transfer switch in 3-phase/4-wire systems, it is often appropriate to switch the neutral. There are two methods by which 4-pole transfer switches switch the neutral pole:

  1. Simultaneously switched neutral, in which the neutral pole is switched on a common crossbar with the phase poles. In this system, the neutral poles of the two sources are never connected.
  2. Overlapping neutral, in which the neutral pole is switched using a separate mechanism. In this scheme, the load neutral is closed to both source neutrals during transition, a period of up to 100 msec.

There are inherent potential failure modes in an overlapping neutral product that don’t exist in a simultaneously switched neutral product:

  1. The overlapping neutral contact is a complex mechanism that introduces failure modes that don’t exist with conventional 4-pole switches.
  2. The overlapping neutral contact does not have the same fault withstand capability as the phase contacts and has not been tested or recognized by any third-party test lab.
  3. Overlapping neutral compromises ground fault detection systems.

Solid neutral is when the neutral form of the generator is solidly interconnected to a service-supplied (utility) neutral.

Q: Since contractors wait until the last minute to provide their fault current and coordination study — and I have no idea whose breaker they will select — is my only option to use the time-based rating?

A: The best approach is to indicate the available fault current at the switch location from the short-circuit analysis. This gives the transfer switch manufacturer higher flexibility in providing a solution to protect the transfer switch

Q: Do these requirements pertain to both electro-mechanical switches and solid state switches? (Typically, the latter is needed for sub-cycle switching.)

A: UL 1008 doesn’t cover solid state transfer switches. Solid state transfer switches are covered under IL 1008S. The tests under UL 1008S are:

  • Normal operation test
  • Overload test
  • Temperature test
  • Endurance test
  • Dielectric voltage-withstand test
  • Withstand
  • Receptacle withstand
  • Short-time current rating test (optional)

Q: In order to specify a time-based rating, is it sufficient to only say, “A time-based rating of ___ amps”?

A: It is best to indicate the available fault current at the switch location from the short-circuit analysis. This language can be used to help when specifying WCR.

Fault-current WCR: UL 1008 WCR must be listed as meeting the requirements for use with protective devices at installation locations under specified fault conditions. WCR shall be based on use of the same set of contacts for the withstand test and the closing test. WCR shall be adequate for duty imposed by protective devices at installation locations in the project under the fault conditions indicated, based on testing according to UL 1008.

Transfer switches shall have a time-duration WCR of at least 0.05 seconds (3 cycles at 60 Hz).

Short-time WCR shall be rated for a duration of 0.5 seconds (30 cycles at 60 Hz).

Transfers switches with withstand ratings only and without closing rating shall not be acceptable. This applies for short-time and time duration WCR ratings.

Q: Where is a list of specific breakers found? On the ATS label? Or ATS operation and maintenance (O&M)?

A: It is on the transfer switch label. It is required to have this list printed on the label. Also, this information is available from the transfer switch manufacturer and typically provided in the submittal phase.

Q: Can you briefly explain 30-cycle ATSs and when and/or why they are required?

A: Short-time current rating: It is a time duration short-circuit WCR test. Durations are set by the manufacturer.

Passing Criteria:

  • Ability to operate the switch and close to the opposite source.
  • No breakage of switch base or any other internal parts.
  • Door must stay secure.
  • Cables must stay connected to lugs without insulation damage.
  • No continuity between the normal and alternate source terminals.
  • Pass a dielectric voltage-withstand test.

And also:

  • Pass a temp-rise test.

Short-time demonstrates that the transfer switch can still carry rated current. This rating is a performance rating, and it can be utilized when the distribution board ahead of the transfer switch is a UL 1558. High short-time WCR simplifies selective coordination strategies, and the coordination is up to 0.5 seconds.

 


Hassan Obeid
Author Bio: Hassan Obeid, Global Technical Advisor for Systems and Controls – Critical Protection, Cummins Power Generation