Knowledge base

AN-FUS-006

Welder Protection


Provides guidelines on using fuses to protect electric welders as per the National Electrical Code..


General
Articles 630-12 and 630-32 of the National Electrical Code requires that electric welders and their supply conductors have overcurrent protection. The Code further requires that each welder have a nameplate which provides information necessary for the selection of the appropriate supply conductors and overcurrent protection devices.

While either circuit breakers or fuses may be used for overcurrent protection, the typically high available fault currents and the need for overall system selective coordination favor the use of current-limiting fuses.

Supply Conductor Protection
For AC transformer, DC rectifier and motor-generator arc welders the supply conductors should be fused at not more than 200% of the conductor ampere rating. For resistance welders the Code allows fusing at up to 300% of conductor ampere rating. In both applications a time delay RK5 fuse such as the Tri-onic® is generally recommended.

Welder Protection
To comply with the Code, AC transformer, DC rectifier and motor-generator arc welders should be fused at not more than 200% of their primary current rating (shown on welder nameplate). Resistance welders should be fused at not more than 300% of their primary current rating. As with supply conductors, RK5 time delay fuses such as the Tri-onic® are recommended. It should be noted that the Code states that a separate overcurrent device is not required for the welder if the supply conductors are protected by an overcurrent device which will satisfy the welder overcurrent protection requirements.

Special Applications
UL class fuses sized according to the Code may not be suitable in some welding applications. High ambient temperatures, high cycle rates and high available fault currents may require the use of Mersen Welder Protectors.

Welder Protectors (A4BX Type 150 or Type 150J) are special purpose limiters which have been designed specifically for welding applications to protect equipment in case of short circuits. They have twice the thermal rating of UL Class fuses yet provide a low clearing I2t . This combination minimizes fuse fatigue and allows effective coordination with upstream devices.

Welder Protectors may be sized closer to welder primary ampere rating than UL Class fuses, hence may allow the use of smaller disconnect switches. Welder Protectors are intended for short circuit protection and are not intended for overload protection. They should never be used as the only protective device on any welder application. Thermal overload protection must be provided in the welder by some other device.