Allen-Bradley 21T-380127-A07 Control Transformer Unit

Module Feature

21T-380127-A07 is a factory authorized control transformer spare part of Allen-Bradley, specially designed to match 905 and 943 series circuit breakers. It is used to supply isolated low-voltage control power for breaker shunt trip coil, auxiliary contact and local indication circuit. The winding adopts insulation treatment, good anti-short circuit capability, stable output voltage under rated load. It belongs to the internal auxiliary accessory of the breaker, drop-in direct replacement, widely used in industrial low-voltage distribution cabinet overhaul.

Working principleThe primary winding connects to distribution power supply, electromagnetic induction transfers energy to secondary winding, output stable isolated AC voltage for breaker control loop. It isolates control circuit from main power circuit, reduce interference and electric shock risk of control side.

Onboard componentsSilicon steel core, primary copper winding, secondary copper winding, insulating paper, terminal block, plastic fixing bracket.

LED indicatorsNo built-in LED indicator, breaker trip and status signal feedback by auxiliary contact.

Hardware self‑diagnosisTransformer itself has no electronic self-check. Abnormality can be judged by no output voltage, overheating or fuse blow of control loop.

Programming softwareNo supporting configuration software.

Electrical & Process Parameters

Model:21T-380127-A07 Brand:Allen-Bradley (Rockwell Automation) Device type:Breaker auxiliary control transformer Applicable host:905 / 943 circuit breaker Function:Isolated auxiliary power supply for breaker trip and control circuit

Mechanical & Structure

Mounting:Internal fixed installation inside circuit breaker frame Protection class:IP20 Housing:Insulating bracket, impregnated winding

Environmental

Operating temperature:-20℃ ~ +60℃ Storage temperature:-40℃ ~ +85℃ Ambient humidity:5~95% non‑condensing Certification:UL, CE Status:Allen-Bradley original breaker spare

Installation

Cut off the main power and control power before replacement. Install transformer to original fixed position, connect primary and secondary terminals strictly according to wiring diagram, check insulation resistance, test trip action after power restoration.

Interface

Primary input terminals, secondary output terminals.

Diagnostic

No secondary output: Winding open circuit, wiring terminal loose. Transformer overheat: Secondary short circuit, overload. Breaker cannot trip: Transformer output voltage insufficient, winding aging.

Product Brief

Allen-Bradley 21T-380127-A07 control transformer, special auxiliary power spare for 905 and 943 circuit breakers.