Microclimate Ultra Fast Fuse Pack (5) - Not to be confused with standard fast-blow fuses fitted to some thermostat brands, these ultra-fast fuses operate fast enough to protect thermostats against large electrical spikes that can occur especially when an incandescent lamp fails. All Microclimate Prime thermostats have these fitted as standard as do all Microclimate Dimming thermostats (B1 and B1ME) manufactured after January 2014.