Schneider · Merlin Gerin · obsolete since 2003
Masterpact M Repair and Refurbishment
The first-generation Masterpact went out of production in 2003 and the OEM answer is a new MTZ in the old chassis. We repair the breaker you already have. Mechanism, contacts, coils, charging motor and the STR trip unit, rebuilt and injection-tested, then flown back to drop into its own cell.
- FramesM08–M63
- Trip unitsSTR28 · STR38 · STR58
- Turnaround2–5 workshop days
- Warranty24 months on overhauls
Repair, don't replace
An obsolete breaker is not a dead breaker
Schneider stopped making the Masterpact M in 2003. Spares followed, and the STR trip units are now out of support. When an M fails, the standard OEM offer is a retrofit: a new MTZ breaker built into the old chassis so it can go back in the switchboard.
That works, but it is a project. The Masterpact M will not go into an NW cradle, so there is no simple like-for-like swap. A retrofit means a new breaker, adapted mounting, rewired secondaries and a commissioning visit, all on the OEM's lead time.
Obsolete describes a supply decision, not the condition of the breaker. The M mechanism and contacts rebuild well. Parts come from our donor breakers, from re-machining to pattern, or from engineered modern equivalents, and the certificate tells you which. The breaker goes back in its own cell, on its own busbars, with the control wiring untouched. No board modification and no new cradle.
- Same frame, same cell, same secondary wiring
- No waiting on a factory retrofit slot
- Fixed price after strip-down, before any repair starts
Common faults
What goes wrong on a Masterpact M
Most M-frame breakers in service are now more than twenty years old. The faults are mostly age, not abuse.
Won't close, or closes slowly
The grease in the mechanism hardens with age until closing is sluggish or the breaker won't latch at all. On the M, the closing coil itself is the other usual suspect.
Won't charge
Worn brushes or a tired gearbox on the charging motor, a failed limit switch, or a fault in the charging handle linkage. Motors are rebuilt, or replaced with an engineered equivalent where the original is no longer available.
Nuisance tripping
An STR unit that trips for no reason is usually end-of-life electronics. Secondary injection testing shows within an hour whether the trip unit or something downstream is at fault.
Hot cradle contacts
Heat at the cradle stabs on a thermal scan usually means eroded or loose cluster contacts. They are replaced as sets and the contact pressure is checked.
Worn contacts
Main and arcing contacts erode with operations and fault clearances. Wear is measured against the OEM limits and the contacts are renewed or dressed.
Broken handles and buttons
Cracked charging handles, missing push buttons and broken internal plastics are common on older units and are replaced as part of the rebuild.
The refurbishment
What we do to an M-frame breaker
- Full strip-down, every part labeled against its position
- Mechanism dismantled, cleaned of old grease and corrosion, worn pivots, springs and latches replaced, re-lubricated and checked for travel and latching
- Main and arcing contacts measured against OEM wear limits and renewed or dressed; arc chutes cleaned and inspected
- Closing coil, shunt trip and undervoltage release tested, and replaced where marginal
- Charging motor and gearbox rebuilt or replaced, charge time checked
- STR trip unit tested. If the electronics have failed, an ETC-12Q or AC-PRO replacement trip unit is fitted inside the original frame and set to reproduce the STR characteristic
- Cradle cluster contacts and secondary disconnects checked if the cradle comes with the breaker
How it is tested
Secondary injection on every protection element fitted, against the published trip curve. Contact resistance per pole, insulation resistance, and mechanical operation through the full charge, close and open sequence. All of it goes on a certificate against the serial number, with before and after photos and a parts list.
Coverage
Masterpact M models we repair
| Frames | M08 to M63, fixed and drawout |
| Breaking classes | H1, H2, H3 |
| Trip units | STR28, STR38, STR58. Where the STR has failed we fit an ETC-12Q or AC-PRO replacement trip unit inside the original frame |
| Exchange units | Tested Masterpact M units held in the UK*, flown out the day a fault is reported. Exchange units |
| Parts sources | Donor breakers, re-machining to pattern, engineered modern equivalents |
| Related ranges | Moeller IZM20 to IZM40 share the Masterpact M mechanism. See Eaton / Moeller IZM. For the later generations, see Masterpact NW & NT. |
*Subject to model and rating.
From your site and back
Turnaround, Express and exchange
Standard
About 3 days air freight each way and 2–5 working days in the workshop, depending on what the strip-down finds. Typically 8–11 working days door to door, plus customs clearance.
How it worksExpress
A 1–2 day workshop turnaround for a site that can't wait. About 7–8 working days door to door, plus clearance.
Exchange units
We hold many tested units in the UK*. If we have your model, a unit ships by air the day you report the fault.
Exchange unitsCustoms
You choose DAP or DDP. We prepare the commercial invoice and repair-and-return paperwork. Your broker confirms the duty treatment.
Shipping & customs*Subject to model and rating.
FAQ
Masterpact M questions
Schneider says the Masterpact M is obsolete. Do we need a new breaker?
No. Obsolete means Schneider no longer makes the range or its STR trip units. The mechanism and contacts rebuild routinely, and a failed STR can be replaced with a modern trip unit inside the original frame. The breaker goes back in its own cell.
Why not just take the MTZ retrofit?
You can, and for some sites it is the right call. It is a bigger job: a new breaker, adapted mounting, new secondary wiring and commissioning, on the OEM's lead time. A repair keeps the existing breaker, cell and wiring, and usually comes back faster.
Where do the parts come from for an obsolete breaker?
Three places: donor breakers we strip for serviceable parts, components re-machined to pattern, and modern equivalents where the design allows, typically coils, motors and trip units. The certificate lists what was fitted.
Will the replacement trip unit match our protection settings?
Send the settings from your drawings or the existing unit. The new trip unit is set to reproduce the original STR characteristic at those settings and injection-tested at them before it ships.
Do you need the cradle as well as the breaker?
Usually just the breaker. If thermal scans show heat at the stabs, send the cluster contacts or talk to us about the cradle, since eroded clusters cause the heat, not the breaker.
What warranty do we get?
24 months on a full overhaul or refurbishment and 12 months on a repair, in writing. Warranty details.
Send us the nameplate
A photo of the nameplate and the fault is enough for a first answer. We reply the same working day with a recommendation and a price for diagnosis and freight.