The R851B/V series line voltage ( up to 240 V a.c.) electronic step controller is designed for cost-effective and precise control of multi-stage applications typically found in electric duct heaters and boilers. Available in four or eight stage models, the R851V can also be used in conjunction with the R810 power switch or R820 SCR power controller, which will result in a fully proportional control of the load. The R851B is specifically designed for electric boiler sequencing.

The R850 can also be used on any type of staging applications found in refrigeration or automation applications.