Schneider Electric · Merlin Gerin
Masterpact Repair & Refurbishment
Masterpact M, NT, NW and MTZ air circuit breakers — 630 to 6300 A — stripped, rebuilt and injection-tested at our Pontefract workshop. Including the Merlin Gerin M range Schneider stopped supporting years ago.
- RangesM · NT · NW · MTZ
- Trip unitsSTR · Micrologic · retrofits
- Turnaround24–48 hrs once quoted
- Warranty12 months
The range
Four generations, one workshop
Masterpact is the most common ACB in UK switchboards built since the early 1990s, which means the fleet spans four generations. The Merlin Gerin Masterpact M (M08 to M63, H1/H2/H3 breaking classes, STR28/38/58 trip units) is obsolete and its electronics are dying of age. The NT and NW that replaced it are still current, but the earliest units are past twenty years old and the mechanism grease has hardened. MTZ is the current range; its faults are the everyday ones — coils, gear motors, secondary disconnects.
We repair all four. Mechanism strip and rebuild, MCH gear motor and XF/MX/MN coil replacement, main and arcing contact sets, cluster contacts on the cradle, secondary disconnect blocks, and the trip unit — repaired, replaced, or on the M range retrofitted with a modern unit that reproduces the original STR characteristic inside the same frame.
Every breaker leaves with a secondary injection test across every protection element, contact resistance per pole, timing and insulation resistance on the certificate.
| M (Merlin Gerin, obsolete) | STR28/38/58 trip unit electronics, hardened mechanism grease, closing coil, charging motor brushes; spares discontinued |
| NT 06–16 | Mechanism stiffening, MCH gear motor, XF closing coil, MN undervoltage release, auxiliary contact blocks |
| NW 08–63 | Hardened grease and slow closing, MCH gear motor, main-contact wear on high-operation duty, secondary disconnect, Micrologic battery and connector faults |
| MTZ1 / MTZ2 / MTZ3 | Coils and gear motor, secondary disconnect, Micrologic X connector and firmware faults, cradle cluster contacts |
| Cradles (all) | Cluster contact erosion and overheating, racking mechanism wear, position switch faults |
Symptoms
Recognise the fault? We repair the cause.
Won't close, or closes slowly
On any Masterpact over ten years old the first suspect is hardened grease in the mechanism. On NW and NT, check the MCH has charged and the XF coil has supply; on the M, the closing coil itself. All repairable. Why an ACB won't close
Won't charge
MCH gear motor, its limit switch, or the charging handle linkage. On the M range the motor brushes and gearbox. Motors are replaced or rebuilt; on obsolete units we engineer a modern equivalent. Why an ACB won't charge
Nuisance tripping
An STR unit on an M-range breaker that trips for no reason is almost always end-of-life electronics — and Schneider no longer supplies them. Micrologic units drift or lose their battery. Injection testing tells us which, in an hour. Injection testing
Hot cluster contacts
Thermography showing heat at the cradle stabs means eroded or loose cluster contacts. Replaced as sets, with the stabs cleaned and the contact pressure verified.
Obsolete and quoted a new board
The Masterpact M doesn't fit an NW cradle, so a like-for-like swap isn't on offer. Repairing the M — with a modern trip unit inside it — keeps the board and the cell. Obsolete breaker repair
Trip unit conversion
STR to a modern electronic unit; Micrologic A to P or H for metering and comms. Done in the workshop, set to your protection settings, injection-tested at those settings. Protection unit replacement
FAQs
Frequently asked
Schneider says the Masterpact M is obsolete and we need a new breaker. Do we?
No. Obsolete means Schneider no longer manufactures the range or its STR trip units; it doesn't mean the breaker can't be repaired. The M's mechanism and contacts are rebuilt routinely, and where the STR electronics are dead we fit a modern trip unit inside the original frame, set to reproduce the STR characteristic. The breaker goes back in its own cell.
Can you repair a Masterpact NW without removing it from the board?
Drawout NW and NT units come out of the cradle and go to the workshop — that's where the bench, the injection test set and the parts are. Coils, gear motors and trip-unit swaps can be done on-site where the site needs it. Fixed units are assessed case by case.
Do you use genuine Schneider parts?
Where they exist, yes — MCH motors, XF/MX/MN coils, Micrologic units and contact sets for NT, NW and MTZ are Schneider parts. For the obsolete M range, parts come from donor breakers, re-machining to pattern, and engineered modern equivalents, and the certificate says which.
Do you support Square D Masterpact in the US?
Yes. Square D-branded Masterpact NW and NT are the same IEC frames. We already air-freight Masterpact units to and from US customers. See repair for US operators.
Get started
Tell us about the breaker
Make, model and rating from the nameplate, plus a description of the fault. We'll come back with a clear recommendation and a fixed price — usually the same working day.
- Workshop & office01977 253063
- Out of hours07496 013884
- Email & photosinfo@circuitbreakerrepair.co.uk
- WorkshopUnit 13–17 Dale Court, Pontefract, West Yorkshire WF9 3FL