⚫ Detects phase loss, low voltage, phase reversal
⚫ 50Hz, 60Hz and 400Hz models
⚫ Automatic or manual reset
⚫ Five year unconditional warranty
MODEL 257
3-Phase Monitor
Shown with optional 8-pin socket

The Model 257 continuously monitors 3-phase power lines for abnormal conditions. When properly adjusted, the
Model 257 monitor will detect phase loss on a loaded motor even when regenerated voltage is present.
This device consists of a solid-state voltage and phase-angle sensing circuit, driving an electromechanical relay.
When correct voltage and phase rotation are applied, the internal relay will energize. A fault condition will de-
energize the relay. When the fault is corrected, the monitor will automatically reset (a manual reset version is also
available).
The Model 257 does not require a neutral connection and can be used with Wye or Delta systems. Voltage ranges
are sufficiently wide to allow for proper adjustment to existing conditions. Both “TRIP” and “NORM” condition
indicators are provided to aid in adjustment and system trouble-shooting.

SPECIFICATIONS
AUTO Reset
MANUAL Reset
B257B
B257BM
257B
257BM
A257B
A257BM
EX257B
EX257BM
B257B-400Hz
B257BM-400Hz
257B-400Hz
257BM-400Hz
EX257B-415V/50Hz
EX257BM-415V/50Hz
A257B-400Hz
A257BM-400Hz
Nominal AC
(phase to phase) 120VAC 208/240VAC 480VAC 380VAC 120VAC 208/240VAC 415VAC 480VAC
Case Color Gray Red Yellow Yellow Gray Red Yellow Yellow
Adjustment range 85-120VAC 160-240VAC 380-480VAC 300-400VAC 85-120VAC 160-240VAC 340-440VAC 380-480VAC
Frequency 60Hz 60Hz 60Hz 50Hz 400Hz 400Hz 50Hz 400Hz
Power consumption 1.4W 2.4W 3.7W 3.0W 1.4W 2.4W 3.3W 3.7W
Transient protection 2500VAC for 10ms
Repeat accuracy ± 0.1% of set point (fixed conditions)
Response time 50ms drop out
Dead band Approximately 2%
Output contacts SPDT 10 amps at 240VAC resistive
Expected relay life Mechanical: 10 million operations Electrical: 100,000 operations at rated load
Operating temp -20° to +131° F
Humidity tolerance 0 - 97% w/o condensation
Enclosure material ABS plastic cover
Mounting 8-pin socket ( **sold separately)
Weight 5 ounces
Agency approvals NONE
UL Recognized* and CSA Certified
*Conditions of acceptability: (1) the 380V and 480V versions must be used with a UL Recognized 600VAC
socket. (2) For use in a Pollution Degree 2 environment (3) To be installed on the load side of branch cir-
cuit protection rated 10A.