Fuse Selection for Two 120Vac Rotating Lights - Page 1
Forum section: electronics / projects

author: jeremypw
author_rank: Newbie
timestamp: October 16, 2014, 01:55:34 pm
content: I'm designing a circuit that will control turning on/off two rotating emergency lights. The specs for the rotating lights are linked below. The motors for each lights is a Synchron brand single-phase synchronous motor that runs at 60 rpm. The lights will be turned on by an arduino through a relay. I'd like to include a fuse in my design. I plan to just put the fuse on the AC side so the operating current should be 0.36x2 = 0.72 Amps. I'm not sure what the initial current draw is but I imagine I need a slow blow fuse at about 2A.What specs should I look for in a fuse?Current Rating?I2t rating?breaking capacity @ rated voltage?http://www.federalsignal-indust.com/product/121s-vitalite-rotating-warning-light/specifications

author: Tandy
author_rank: Frequent Contributor
timestamp: October 16, 2014, 05:26:22 pm
content: Yes as you have an inductive load (motor) and a incandescent bulb there will be a high inrush current so a slow blow fuse (T) would be your best bet.A 1A fuse should be sufficient, you are not going to exceed the breaking capacity in this kind of application so a standard 20mm glass low breaking capacity fuse would be fine.

author: jeremypw
author_rank: Newbie
timestamp: October 16, 2014, 07:41:10 pm
content: Thanks Tandy - Under what circumstances will I need to worry about the breaking capacity, and I2t values?

author: Tandy
author_rank: Frequent Contributor
timestamp: October 16, 2014, 08:56:19 pm
content: Breaking capacity also known as the interrupt rating is the maximum current a fuse can safely break at the rated voltage. In a short circuit condition, a fuse may receive an instantaneous overload current many times greater than its normal operating current, the wire melts and an arc is formed between the two ends of the wire until the gap is big enough to extinguish the arc. The heat generated can potentially result in the fuse cartridge cracking, ignition or even explosion. The worst case scenario in a high voltage situation is that the electricity would arc across the two fuse terminals therefore allowing the current to continue to flow even when the fuse has blown.To solve these problems a high breaking current fuse is used, this is typically a sand filled ceramic fuse this extinguishes any arc and reduces the amount of heat transferred to the cartridge of the fuse.Typically the higher the power the more of a problem this is as the arc can take longer to extinguish. In this example we are talking about a relatively low voltage of 120VAC and the typical breaking capacity of a 20mm glass fuse is 35A. While there are no guarantees your 1A glass fuse is unlikely to burst or catch fire. It is after all a secondary fuse that is intended to protect the switching relay, if the fuse had not completely failed by the time it reached its 35A limit the main circuit protection device on your wiring board would blow/trip.There is no harm in using a ceramic high breaking capacity fuse in place of a glass one if you want the security of knowing it is more durable during a fault. The only down side is you can easily see when the wire has melted in a glass fuse but you have to use a tester on a ceramic one.The melting integral or I2t value is the amount of energy necessary to heat up and melt a wire or fuse element. A slow blow fuse has a higher I2t value to enable it to handle the initial peak of higher current.

author: jeremypw
author_rank: Newbie
timestamp: October 17, 2014, 02:26:45 am
content: Thanks, excellent explanation of breaking capacity. I'm having a difficult time understanding I2t enough to apply it in selecting a fuse. I'm looking at the digikey catalog and have filtered down to just slow blow fuses but the value of I2t ranges from 2.4 to 28. Does the value matter much in this case? Under what circumstances would I need to be concerned with the value? How do I determine what value is needed in general?Digikey Fuses

author: SeanB
author_rank: Super Contributor
timestamp: October 17, 2014, 07:49:38 pm
content: The larger rating the longer it takes the fuse to blow at moderate overload, like double the current. This is for motor load and light or transformer inrush protection, which can have a switch on transient of 2-5 times the running current for a few cycles. Thus you can choose any fuse for your application from the selection, as they all will provide the same holding current, and all will fail in under 2 cycles at short circuit currents. The higher I2T versions just will take longer to blow with moderate overloads, typically a second or ten more. Just buy any that are 1A, are the right size and are in stock and cheap. If you are worried buy a pack of 5 and keep spares in the casing.