Thomson 3 Position Test Switch — Portable Generator & Load Bank Connection

Overview#

The Thomson 3 Position Test Switch (3PTS) provides a quick and safe solution for connecting a portable generator or load bank to your emergency power system. The three-position design — Normal, Test, and Off — allows facilities to test their emergency power systems per NFPA 110 requirements or connect temporary power during planned maintenance without modifying permanent electrical connections.

The 3PTS is available from 100A to 1200A in 3-pole configurations with voltage options from 120/208V through 347/600V. All models are UL 1008 Supplement SB and CSA C22.2 No. 178 listed. NEMA 1 indoor and NEMA 3R outdoor enclosures are available.

How it works#

The 3PTS operates as a manual three-position switch:

Normal position — load is connected to the permanent utility/generator source through the facility's existing ATS
Test position — load is transferred to the portable generator or load bank connected via the test switch's inlet connectors
Off position — both sources are disconnected, providing a safe isolation point for maintenance

The break-before-make switching mechanism prevents paralleling of permanent and portable sources at any point during position changes.

Typical applications#

NFPA 110 load testing — connect a load bank to verify emergency generator capacity without affecting facility operations
Planned maintenance — connect a portable generator to maintain power while the permanent generator is serviced
Disaster recovery — provide a pre-wired connection point for rental generators during extended outages
Commissioning — test new emergency power installations before permanent generator delivery

Our service experience#

We install Thomson Position Test Switches alongside ATS installations in healthcare, data center, and commercial facilities across the Bay Area. The most common configuration is a 3PTS matched to the facility's ATS amperage — for example, pairing a 1200A 3PTS with a TS870 or TS880 ATS to provide a permanent load bank connection point for annual NFPA 110 testing.

The NEMA 3R outdoor enclosure variant is popular for facilities where the test connection needs to be accessible from the generator parking area or loading dock, keeping portable generator cable runs short and manageable.

Frequently Asked Questions

What is a Thomson 3 Position Test Switch used for?

The Thomson 3PTS provides a safe, permanent connection point for portable generators and load banks to your emergency power system. It allows NFPA 110 load testing and portable generator connection without modifying permanent wiring or opening electrical enclosures. The three positions are Normal (utility only), Test (portable source connected), and Off (both disconnected for maintenance).

What amperage ratings are available?

The 3PTS is available in 100A, 200A, 400A, 600A, 800A, and 1200A ratings in 3-pole configurations. Voltage options include 120/208V, 120/240V, 277/480V, and 347/600V. Both NEMA 1 indoor and NEMA 3R outdoor enclosures are available.

When should I specify a Position Test Switch vs a Quick Connect Panel?

The Position Test Switch (3PTS) is a switching device with three selectable positions — it actively switches between utility and a portable source. The Quick Connect Panel (QCP) is a connection-only panel with cam-lock receptacles — it provides a plug-in point but does not include switching. Use the 3PTS when you need a code-compliant transfer mechanism for portable generators. Use the QCP when the facility already has a separate transfer switch and only needs a connection point.

Key Terms

3-position test switch

A manual switching device with three positions: Normal (utility only), Test (portable generator/load bank), and Off (both sources disconnected). Provides a safe, code-compliant connection point for testing emergency power systems without modifying permanent wiring.

UL 1008 Supplement SB

UL 1008 supplementary certification for manual transfer and test switching equipment. Covers devices that provide safe connection points for portable generators and load banks in emergency power systems.