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 * Three Reasons to Choose UL 1008 Transfer Switches


THREE REASONS TO CHOOSE UL 1008 TRANSFER SWITCHES


Transfer switches are critical to powering life-safety and legally required
systems as well as mission-critical loads. UL1008 – Standard for Safety –
Transfer Switch Equipment specifies the safety and endurance tests for verifying
that transfer switches offer the highest levels of reliability and power
availability. This article highlights important aspects of UL 1008 testing.

Highlights of UL 1008 Electrical Testing

Because a transfer switch is always connected to a power source, it must be
capable of carrying its rated amount of current continuously. When faults occur,
a transfer switch must carry the resulting current until it is cleared by a
circuit breaker or fuse. If a transfer switch instead opened when faults
occurred, it would leave loads without power even when power is restored to the
facility. For the same reason, a transfer switch must also be able to reliably
close on fault currents.

Manufacturers design generator transfer switches to provide specific service and
fault current capabilities. To verify them, UL 1008 tests the ability of
transfer switches to withstand and close on fault currents. The procedures
required to verify corresponding ratings and functionality under fault
conditions include the Withstand Test, the Closing Test, and the Dielectric
Voltage Withstand Test.

Withstand Test

A transfer switch with closed main contacts is subjected to a short-circuit
current of 20 to 50 times its service rating. More specifically, switches rated
up to 100, up to 400, and greater than 400 Amps must respectively withstand
minimum currents of 5,000 Amps, 10,000 Amps, and current at least 20 times the
nominal switch ampacity (but not less than 10,000 Amps) until the current is
cleared by an upstream fuse or breaker. The power factor must not exceed 0.50
for currents up to 10,000 Amps, 0.30 for currents up to 20,000 Amps, and 0.20
for currents exceeding 20,000 Amps.

Closing Test

Transfer switches must also be able to close on fault currents until cleared by
an upstream protective device. Consequently, the withstand test is repeated,
this time starting with the main contacts open. During this test, the contacts
must close on and withstand a current with the same electrical parameters used
for the Withstand Test.

Dielectric Voltage Withstand Test

Following these tests, an ac current of twice the rated voltage (but not less
than 900V) must be applied to a transfer switch while voltage is measured at six
types of locations within the device.

Passing Criteria

The switch must present the following conditions when the tests are complete:
• ability to close on untested contacts of the opposite power source circuit
• operability using whatever automatic or manual controls are provided
• no continuity between the terminals of the normal and alternate power source
circuits
• no opening of doors on transfer switch enclosures
• cables cannot pull away from lugs and connectors

Optional Short-Time Tests

The aforementioned electrical tests require switches to carry a fault current
until cleared by a protective device. However, many facilities employ selective
coordination schemes using breakers that must withstand fault currents for
specific durations. UL 1008 gives equipment manufacturers the option of
demonstrating that a transfer switch can withstand and close on currents for
times ranging from 0.1 to 0.5 seconds. If successful, manufacturers can mark a
switch with corresponding Short-Time Ratings.

For more information about UL 1008 electrical rating tests, review the ASCO
Power Technologies document entitled UL 1008 Transfer Switch Withstand and
Closing Ratings. ASCO’s Selective Coordination Basics provides insight into
coordinating overcurrent protection devices to minimize the scope of outages.

Performance Tests

Tests with profound implications for switch safety and longevity include the
Overload Test, Endurance Test, and Temperature Rise Test.

Overload Test

An overload test of a transfer switch requires cycling electrical load between
power sources while the switch carries the overcurrents prescribed in Table 1.
According to switch ampacity, it must carry these currents for the quantities of
cycles listed in Table 2.


Using the values in the tables, a nominally rated 800 Amp ac transfer switch
serving Total System Loads must be subject to 4800 Amps at a power factor
between 0.4 and 0.5 for 50 operating cycles at a rate of one cycle every 4
minutes.

Temperature Rise Test

Following overload testing, coils and heating elements in the switch are
energized while the transfer switch carries 100 percent of its rated load until
temperatures in the device equilibrate. The resulting temperatures are then
compared to maximum temperature criteria for more than a dozen types of
components and insulating materials specified by UL 1008.

Endurance Test

Endurance tests involve operating transfer switches with (and sometimes without)
current at specified power factors. They must carry specified amounts of load
for the quantities of cycles shown in Table 3.

If the manufacturer wishes to mark the switch for use with only “Optional
Loads”, then the reduced quantities of cycles shown in Table 4 may be used. Such
switches cannot be used for life-safety or legally required loads.

Passing Criteria

If a switch passes the Temperature Rise Test, the endurance tests do not result
in evidence of mechanical or electrical dysfunction, and the switch passes a
subsequent Dielectric Voltage Withstand Test, it will have satisfied UL1008
requirements for performance. For additional information regarding UL 1008
performance tests, review the ASCO Power Technologies’ document entitled
Performance Testing for Transfer Switches.

Significance of UL 1008 Tests

The UL 1008 standard specifies rigorous testing requirements for transfer switch
equipment. Three resulting benefits of using an UL 1008-Listed transfer switch
include:

1. Withstand and closing tests that place severe stresses on the transfer switch
2. Endurance testing the surpasses the operating conditions that most transfer
switches will ever see
3. The assurance that comes with subjecting an entire transfer switch device,
and not just a subset of its components, to severe conditions

As a result, the UL 1008 standard reflects safety, reliability, and performance
standards that are necessarily severe for devices that will be used to power
life-safety and critical loads. Here’s why.

In the earlier example, an 800 Amp ac switch serving Total System Loads required
overload testing at 4800 Amps and a power factor between 0.4 and 0.5 for 50
operations at four cycles per minute. Likewise, the Endurance Test for the same
switch requires 1000 cycles without current and 2000 cycles with current, with
half of those operations at 200 percent of rated current.

These parameters far exceed the conditions that most facilities will ever see.
It is worth noting that all of the tests are executed using a complete transfer
switch, including its transfer mechanism, controls, components, wiring, and
enclosure. The extent and severity of testing required for a UL 1008 listing
verifies that transfer switches can withstand switching duty under the most
severe conditions and reliably provide many years of service.

References

Underwriters Laboratories. UL 1008 - Standard for Safety – Transfer Switch
Equipment. 2014.

ASCO Power Technologies, Inc. UL 1008 Transfer Switch Withstand and Closing
Ratings. 2019.

ASCO Power Technologies, Inc. Selective Coordination Basics. 2018.

ASCO Power Technologies, Inc. Performance Testing for Transfer Switches. 2020

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