Polyamides suit heavy-duty circuit breakers

MCCBs are heavy duty circuit breakers, mostly used in industrial and commercial buildings, and operate at far higher electrical ratings than miniature circuit breakers, MCBs, used in domestic housing.

DSM is the first engineering plastics supplier to offer a full portfolio of materials for MCCBs. It has solutions for all the separate housing components, as well as for the functional internal parts in the arc extinguishing chamber, which are critical components in MCCBs, resulting in a very safe and cost-effective solution. The results from the latest research confirm the advantages of DSM engineering plastics, and have given a strong indication of the extensive possibilities they offer in this growth market.

The company has been collaborating for over a year with ERDA (Electrical Research & Development Association), a co-operative research institution created by the Indian electrical industry and utilities to cover basic research in the field of electrical applications. ERDA has helped DSM improve its understanding of the thermo-mechanical and electrical performance of thermoplastics in arc chutes.

DSM has been a pioneer in replacing thermosets in Low Voltage SwitchGear (LVSG) applications such as MCBs for over 35 years. With its most recent developments in high-performance, halogen-free flame-retardant materials, and the support of this independently-verified performance data, the company intends to repeat its success in MCCBs. It is in a strong position to demonstrate the benefits of its highest performing grades in the arc chute. Selected DSM partners, such as Jiangsu Phono in China, are already commercialising the concept.

Research at ERDA has confirmed that arc chutes injection moulded in thermoplastics such as halogen-free UL94 V-0 flame retardant types of DSM’s Stanyl polyamide 46 could easily withstand the thermo-mechanical and electrical stresses developed during short-circuit in MCCBs with breaking capacities up to 25kA. But the same concept could potentially be used in MCCBs with much higher breaking capacities. In addition, polyamide 46 provides increased safety margins over thermosets, even in parts with far lower wall thicknesses.

The primary function of the arc chute is to provide final cooling of the arc, in order to help avoid an arc re-strike, and preserve the integrity of the contacts to increase the service life of the MCCB. Until now, arc chutes have been made in thermoset composites (DMC/BMC). The production process is longer than with thermoplastics, reject rates are generally higher, and the materials cannot be melt-recycled. Thermoplastic versions can also be designed with increased functionality built into them, compared to thermosets.

DSM’s high-performance, high-temperature resistant Stanyl polyamide 46 has an additional advantage: it emits gases when exposed to high temperature and UV radiation from the plasma generated by the arc, thus providing extra help in rapid cooling of the arc, reducing carbonisation and contact pitting.

In their research project, DSM and ERDA looked at the performance of various flame retardant aliphatic high temperature polyamides in short-circuit and electrical endurance tests on a single pole MCCB, rated up to 25kA breaking capacity and up to 100A nominal current. Materials included Akulon PA66, which has a melting point of 255°C and a maximum continuous-use temperature of 180°C; and Stanyl PA46, which has a melting point of 295°C and a maximum continuous-use temperature of 230°C.

The researchers concluded that thermoplastic arc chutes could withstand transient as well as normal conditions, and also helped to decrease the arcing time and energy dissipated in the MCCB. As a result, there was far less carbonisation than with thermoset arc chutes, helping to improve the long-term performance of the MCCB.

The experiments confirmed that flame retardant glass filled Stanyl arc chutes gave the best results in short-circuit tests. Even though unfilled grades could release far more off-gasses, and so were better than filled ones for arc voltage and carbonisation, the reinforced Stanyl showed much higher thermo-mechanical stability and lower degradation. It is therefore seen by DSM as the best choice, considering the long life expectancy and performance repeatability required from MCCBs in the field.

The two partners now plan to take their cooperation further with a third partner, engaged in manufacturing of instrument transformers. Together, they will research the design and development of challenging switchgear applications that have still not been converted from thermosets to thermoplastics.

Alexis Ponnaradjou, Vice President, Global Research and Technology, Engineering Plastics, DSM said: “We were very confident from the outset that our cooperation with ERDA would lead to innovative solutions on performance and sustainability, and I am happy to say that the results from our joint research have shown that our confidence was well-founded.”