AB 1333 Phase Loss Protection: External Overload Wiring
AB 1333 drives lack internal phase loss protection. Wire an external overload block per user manual, derate for single-phase 220V, and verify 575V supply.
The Allen-Bradley Bulletin 1333 adjustable-frequency drive still appears in pulp-and-paper, HVAC retrofit, and pump stations long after newer PowerFlex platforms took the catalog. A recurring field question is phase-loss protection: the 1333 does not expose a programmable phase-loss parameter, and no late firmware revision adds one. Plants that need NEC 430 / UL 508A-aligned motor branch protection must implement the function externally.
Factory literature pushes overload and phase-loss devices outside the 1333 power section—ENABLE wiring is the consolidation point.
Why the drive is silent on phase loss
Phase loss opens one incoming line while the inverter may continue to run. The DC bus becomes asymmetric, the motor sees heavy negative-sequence current, and winding or rectifier damage can precede any overcurrent trip by tens of seconds. Phase imbalance is a milder voltage deviation; the 1333 monitors neither condition internally. The platform was designed as a building-block drive whose protection philosophy lived in the MCC: overload, phase loss, reversal, and ground fault were expected upstream or downstream of the module.
Three required protection layers
| Layer | Device example | Action |
|---|---|---|
| Incoming phase loss / sequence | Bulletin 813S or equivalent | Open ENABLE within ~200 ms of lost phase |
| Branch short-circuit / ground fault | MCCB or fuses per NEC 430.52 | Sized to motor FLA, not drive marketing amps |
| Output thermal overload | Bulletin 193 / 140M class | Carries motor current; NC aux in ENABLE |
Wiring the overload per the factory topology
Mount the overload on the load side of T1/T2/T3 so it carries motor FLA continuously. Land the normally closed auxiliary (often terminals 95–96) in series with the drive ENABLE/STOP permissive string (commonly around control terminals 18–19 depending on revision). Use the normally open auxiliary for PLC fault indication if desired. Set the overload to motor nameplate FLA (and service-factor rules), never to the drive’s larger continuous rating.
T1/T2/T3 (drive) -> OL line side OL load side -> motor leads OL NC (95-96) -> series in ENABLE permissive OL NO (97-98) -> optional PLC fault input
Single-phase 220 V feeds on a three-phase 1333 demand heavy derating—or a voltage-class replacement.
Single-phase operation and voltage class traps
Landing two hots on any two of L1/L2/L3 will sometimes spin a motor, but rectifier ripple and bus capacitor stress rise sharply. Continuous output capability commonly falls near half, and available shaft power can drop toward roughly one-third of nameplate for a 230 V class unit. Increase input fuse sizing per the elevated line current. Critically, a 575 V class 1333 will not wake up on 220 V single-phase: pre-charge and control supplies never reach release thresholds. That dark front panel is undervoltage inhibit, not a mysterious logic fault—provide the correct three-phase voltage class or replace the drive.
| Parameter | 3-phase | 1-phase 220 V (230 V class) |
|---|---|---|
| Continuous output current | 100% rating | ~50% rating |
| Typical available HP | Nameplate | ~1/3 nameplate |
| DC bus ripple | ~5% | ~25% |
ENABLE chain and phase-loss trip test
- Confirm OL set to motor FLA; clamp-check current at reduced speed
- Open the OL NC path and prove ENABLE drops the inverter output
- Simulate upstream phase loss on the voltage monitor and time the trip
- Measure control electronics voltage under load, not only at idle
- Verify nameplate voltage class matches the supply before blaming power devices
If the line is migrating, replace drive and motor protection together rather than orphaning a 1333 behind modern breakers with undocumented ENABLE strings. Spares and cutover hardware should follow the plant’s control platform standards even when the power section remains legacy.
About the Author
Mark Townsend | Senior Automation Engineer – Allen-Bradley Systems
Mark Townsend is a senior automation engineer with more than 18 years on Allen-Bradley platforms spanning ControlLogix, CompactLogix, and legacy SLC-500. His day-to-day work is RSLogix / Studio 5000 logic and FactoryTalk View HMI bring-up on aging and mixed fleets.
