Selecting Fuses for a Temperature Control Loop

A customer asked recently: what type of fuses should I use with my new fan and closed-loop temperature control system?

First, we always recommend you consult with an electrician or electrical engineer.  That said, you need a branch protection fuse, but they don't blow quickly enough to protect your silicon controlled rectifers (SCRs).  In addition to a branch protection fuse, you will want to protect your SCR with a semiconductor fuse, which open in tenths of a millisecond (vs. ~1 minute). 

Protection of systems for controlling electric heaters require two types of fusing to protect both the system and the SCRs.

Branch Circuit Fuses are required by code to protect against fire/damage when heaters or wires short. However, with such a short, the branch circuit fuses do not respond quickly enough to protect the SCRs from damage. A branch circuit fuse must open in one minute at 125 percent of the fuse’s nameplate rating.

Branch circuit fuses are selected with a rating at a minimum of 125 percent of the connected load. Go to the next size larger if the calculation results in a fuse size that is not available. When changing to smaller wire gauge or splitting up the load, it may be necessary to add additional fusing for the smaller wire. See the Tap Rule in the NEC 240.21.

Semiconductor Fuses are a specialty fuse, sometimes called a supplemental fuse, and are only used to protect the SCRs. They can open in tenths of a millisecond on a high fault current short. Semiconductor fuses are not legal for branch circuit protection; as they do not have a defined overload rating.

Semiconductor fuses must be rated at a minimum of 125 percent for the RMS rating, but also must be rated in current squared multiplied by time (I2T). Semiconductor fuses may be sized larger than the connected load calculation. Semiconductor fuses supplied with power controllers are rated at 125% of the rated current capabilities of the SCRs. Since the supplemental fuse is meant to protect the SCR, this is perfectly acceptable.

To protect the SCR, the I2T rating of the fuse must be at or lower than the I2T rating of the SCR.

Some commercial products come standard with semiconductor fuses, while others must be ordered separately.

Combination Fuses are available that combine branch circuit and semiconductor fusing in one package.

To speak with an expert about fuses, or for guidance, IES can help. Contact us today.

The original article was contributed by Larry Crane of Watlow Electric Company.

IES Technical Sales is a manufacturer's representative and distribution firm which focuses on technical applications involving vacuum, thin film/plasma, temperature, and flow. For more information, contact us.