Description

Key Technical Specifications

Parameter	Specification
Full Model	MT-91-ARCFPA / MT-91-ARC FP A
Manufacturer	ABB
Product Type	Arc-flash protection relay
Auxiliary Power	24 VDC ±20% (some listings cite 18–30 VDC; verify on nameplate)
Detection Method	Optical sensing (approx. 180–700 nm) + overcurrent detection
Response Time	≤10 ms typical; some listings claim <5 ms — verify on nameplate
Light Sensor Inputs	Up to 8 fiber-optic sensors via expansion (configuration-dependent)
Current Input	Via external CTs; distributor listings cite approx. 0.5–100 A range
Output Relays	4× NO relay outputs, 250 VAC / 30 VDC, 5 A (distributor data)
Communication	RS485, Modbus RTU
Mounting	35 mm DIN rail
Operating Temperature	-20°C to +70°C (distributor data); other listings vary
Isolation	2 kV galvanic isolation between inputs, outputs, and power (distributor data)
Certifications	CE / UL / IEC references appear in distributor listings; confirm on nameplate

Product Introduction

The MT-91-ARCFPA is positioned as a compact arc-flash protection relay for medium- and low-voltage switchgear. Its job is straightforward: detect the intense light of an arc flash, confirm the fault with overcurrent information where required, and trip the breaker before incident energy escalates.In practice, this type of relay is used where switchgear aging, maintenance exposure, or high fault levels make arc-flash risk unacceptable. It typically connects to fiber-optic sensors inside cable compartments, busbar chambers, or breaker cells, and uses relay outputs to trip breakers or send alarms. Communication is usually Modbus RTU over RS485 for SCADA integration.One caution: this exact part string is not easy to validate against ABB’s current public product pages. It is commonly sold through surplus and distributor channels, and some listings describe it differently — as a control panel, PLC, or even a drive power module. Those descriptions should not be trusted without a nameplate photo.

Quality SOP & Tech Pitfalls

The Lab Report (SOP)
We verify the nameplate first: model, revision, serial number, auxiliary voltage, and relay ratings. Then we check mechanical condition — DIN-rail clips, terminal blocks, fiber-optic ports, RS485 connector, and signs of moisture or overheating. Functional testing includes auxiliary power-up, self-diagnostics, optical input simulation, overcurrent input verification, relay output timing, and Modbus RTU communication. We record trip timing, relay contact resistance, insulation checks, and LED/display behavior. Test photos and a basic test report can be provided.The Engineer’s Warning (Pitfalls)

Do not trust distributor descriptions blindly. Some listings call this a PLC, control panel, or ACS880 power module. That is inconsistent with the arc-flash relay description and must be cleared by the nameplate.
Auxiliary voltage must be confirmed. If the unit is 24 VDC and you apply 110/220 VAC, it will be destroyed.
Trip logic must match the protection philosophy. Light-only tripping is fast but more prone to nuisance trips; light + overcurrent is safer but depends on correct CT wiring.
Fiber-optic sensors are not generic. Sensor type, loop/radial arrangement, and channel count must match the relay’s configuration.
Response-time claims vary. Some sources say <5 ms, others ≤10 ms. Do not use either number for safety calculations until confirmed by the OEM label or test report.

Installation & Configuration Guide

Pre-Installation
- Obtain a maintenance window and confirm arc-flash protection bypass rules.
- Photograph the existing relay: nameplate, wiring, sensor layout, setpoints, and relay assignments.
- Verify auxiliary supply voltage and CT ratios.
Removal
- ⚠️ De-energize or place the protection channel in a safe state per site procedure.
- Label fiber-optic sensors, CT inputs, relay outputs, and communication wiring.
- Remove the relay and store it in an anti-static bag.
Installation
- Confirm the replacement nameplate matches MT-91-ARCFPA.
- Mount on 35 mm DIN rail.
- Connect fiber-optic sensors, CT inputs, trip relays, alarm relays, and RS485 wiring exactly as labeled.
- Verify auxiliary power polarity and voltage.
Power-On & Testing
- Check self-diagnostics and LED status.
- Verify optical sensor continuity and fault indication.
- Test overcurrent input scaling.
- Perform controlled trip tests before returning the switchgear to service.
- Confirm Modbus RTU address, baud rate, and SCADA mapping.

Compatible Replacement Models

Replacement	Compatibility	Notes
ABB MT-91-ARCFPA (exact nameplate match)	✅ Preferred	Match model, revision, voltage, and sensor configuration.
ABB REA 101 / REA extension modules	⚠️ System-level substitute	REA family is a documented ABB arc-flash protection system, but it is not a direct drop-in replacement.
ABB TVOC-2 Arc Guard System	⚠️ Different architecture	Fast optical arc-flash mitigation, but wiring, sensors, and integration differ.
Other ABB/IEC arc-flash relays	⚠️ Engineering review required	Requires sensor, CT, trip logic, and communication re-validation.

Frequently Asked Questions (FAQ)

Is MT-91-ARCFPA a PLC or an arc-flash relay?
Distributor listings are inconsistent. The most credible product descriptions identify it as an arc-flash protection relay, but some surplus pages mislabel it as a PLC, control panel, or drive power module. The nameplate must decide.What voltage does it use?
Most listings indicate 24 VDC auxiliary supply, but this must be confirmed on the physical nameplate before wiring.Can it trip a breaker directly?
It provides relay outputs rated for trip/alarm use, but the actual breaker trip circuit, auxiliary voltage, and protection coordination must be verified by a qualified engineer.Does it support Modbus?
Yes, RS485 with Modbus RTU is listed in distributor data.Is it still in production?
Uncertain. It appears mainly in surplus and distributor inventories. Treat it as legacy/surplus unless ABB confirms current availability.What should I send before ordering?
A clear nameplate photo, existing wiring diagram, sensor type and count, CT ratio, auxiliary voltage, relay assignment sheet, and any observed fault codes.Can you guarantee it will work in my switchgear?
No. Arc-flash protection is safety-critical. We can verify model, revision, basic function, and relay timing, but final acceptance must come from your commissioning test and protection study.