Merlin Gerin STR Trip Unit

The first Masterpact M ACB's to roll off Merlin Gerin's production line back in 1986 had the ST type trip units fitted, which gave the purchaser various models to choose from dependent on the designated role of each ACB. There were ST008 (dummy) trip units for switch disconnectors, ST108i for basic instantaneous protection, ST208D for standard distribution type functionality, ST318S & ST418S for selective protection through to ST608U's for universal protection.

However, these were superceded in the early 90's by the STR type of trip unit which gave gave a better range of selectable protection settings, and also was an upgrade of the older tripping technology. The manufacturer, Merlin Gerin (Schneider Electric Ltd) found at this time that there was a potentially dangerous quality problem on the older ST type of trip unit for a large batch of units and launched "Project Spring" which involved trying to track down every affected unit on a world wide basis (an impossible task through all the various distribution channels) and replaced them free of charge. Even though the name may have sounded like it was a mechanical problem with a spring of some type, it was actually electronic componentry on one of the PCB's. Not all of the affected units were found so there may still be units out on the networks with an inherent problem.

Meanwhile, worldwide sales of the Merlin Gerin Masterpact M (also badged as the Square D Masterpact MP in the United States) including the newer range of STR trip units went from strength to strength. However, Schneider Electric could never quite completely throw off the quality issues surounding their electronic trip units, and units progressively started to fail over time due to a low load issue (less than 20% load) which made the units trip out sporadically. Perhaps fortuitously, perhaps as a result of the issues with the STR trip units, Schneider Electric launched their new Masterpact NW & NT ranges to supercede the Masterpact M with a completely new type of electronic trip unit (the Micrologic range).

By 2003, when the last Masterpact M's were manufactured, Schneider Electric had made a conscious effort to try and limit their future exposure and went on to insist that any replacement STR trip unit was fitted by a Schneider Engineer - insisting to the market initially that the issues were to do with lack of expertise in fitting substitute units. As most that failed were factory fitted in the original production phase seems to have escaped their argument.

As part of the obsolescence program in 2003, Schneider Electric rationalised their range of available replacement trip units down to the STR58U - which would also replace the STR18I, STR18M, STR28D and STR38S units - and the STR68U programmable unit. Unfortunately, though, the rate at which STR type trip units are failing is on the increase. Schneider has since issued a statement to say that the problem is caused by a faulty batch of thermistors which due to the growth of dendrites on the component under certain atmospheric, voltage and current conditions can cause the unit to measure a higher temperature than is actually there thus causing the unit to trip. Symptoms of this problem are that the "Red Button" on the face of the STR unit will be pushed out indicating a trip but there will be no fault indication.

Whilst replacing a faulty STR unit with a brand new STR58U may temporarily "fix" the spurious tripping issue on a specific breaker, there is no guarantee how long each trip unit will last before it starts to misbehave. In addition, due to their insistance that all replacements are fitted by Schneider Technicians, the process can be very expensive to carry out. Over the last few years the delivery lead time on the STR58U has been anything up to 28+ working weeks delivery in the UK - and that was before the Service team would even start to schedule the work.

Schneider Electric does now offer an "alternate solution" to this problem, which is by providing a retrofit kit for the withdrawable types of Masterpact M to replace it with a modified Masterpact NW ACB. Unfortunately, this means that standard NW withdrawable ACB's will not fit into the retrofit chassis so Customers are limited in their ability to cope with maintenance / emergency swap-outs. It is also another service that Schneider Electric insist on providing as a fitted service, so the cost can be quite prohibitive and, in my humble view, is a "sledgehammer to crack a nut" solution as the actual Masterpact M ACB's are usually still in pretty good condition when they are changed - meaning that the Customer pays for changing part of the equipment that is still good for a number of years service, just to cope with the trip unit issue.

Until recently there was no other real alternative. However, in early 2010, building on the decade or more of success of their renowned ETC-12 range, Satin American released the ETC-12 Q Series Electronic Trip Unit, which was designed specifically to fit the form of the ST and STR type trip units on Masterpact M ACB's, and provides a far greater functional set as standard than the STR range without any of the spurious tripping issues long associated with the Schneider unit.

More details on the Satin American ETC-12 Q Series unit.