Knowing When to Use a Time Delay Fuse

Learn why you should invest in a time delay fuse.
  • Beginner

A time-delay fuse is a special type of fuse which is designed for specific applications. Fuses are an important part of every electrical system as they protect the user from electrocution and fire. The size of the fuse is carefully chosen to suit the requirements of the appliance or circuit.

If more power is being drawn than there should be, the fuse blows. Traditional fuses use a mechanism similar to a light bulb. A thin wire is used in the fuse, which is the right size for the amount of current being drawn. If any more is drawn then the fuse will burn out and blow, resulting in a dead circuit.

Purpose of a Time Delay Fuse

A time-delay fuse is also sometimes known as a slow blow fuse. The purpose of this type of fuse is to allow a surge in electricity for a short time before the fuse actually blows. To many people, this sounds like a bad idea. However, it's beneficial and actually much safer for certain types of electrical appliances.

These time-delay fuses are designed for specific applications and it's not normally possible to fit a different fuse inside the receptacle.

How They Work

Part of the fuse works like a normal fuse that has the burnout link, but in addition to this, there is a short connector that has a spring attached to it. This spring is soldered onto the fuse wire. These can cope with an overload of 100 or even 200 percent of the normal load without causing any harm to the elements.

If the overload remains for a long time, then this will cause the solder joint to melt. This will, in turn, open the contact.


An electrical motor takes a lot more electricity to start than it does to continue operating. This means that a slow blow fuse is a great way to protect the circuit.

Fluorescent Lamps

A fluorescent lamp will also consume much more power when it is first switched on. This will then decrease over time when it is left on. This is why a slow blow fuse can be beneficial in these cases. Anything that has a coil or starter motor inside will consume more power when starting up than when it's running normally.

Why Is a Slow Blow Fuse Safer?

If it wasn't for slow blow fuses, then you would need to use a higher rated fuse to cope with the initial load of the light or motor. The downside of getting a higher rated fuse is that it would also mean that the fuse is too big for the normal load of the light, and risk overheating during a malfunction.

This could potentially be dangerous because it could mean that the fuse will not blow when the circuit is overloaded. A slow blow fuse will only allow an overload of a specified amount, for a specified amount of time. Anything that goes over those specs will result in it blowing and cutting off the circuit.

Knowing When to Use a Time Delay Fuse

Can I replace a normal fuse with a time delay fuse?

The wires that connect fuses are different when fuses are different. Normal fuses, or fast-acting fuses, have a thin wire.

Time delay fuses have a thicker wire that is actually coiled. Though both are fuses, they are not interchangeable.

What's the difference between a fuse and a time delay fuse?

A time delay fuse works based on the time it takes for the current heat to build up and get the ruse element hot enough. Standard fuses, or fast-acting fuses, work immediately.

How do you know if a time-delay fuse is blown?

When fuses blow, there are visual indicators that they need to be replaced. There will be a gap in the wire or you may even see a burn mark or metal mark on the glass tube surrounding the fuse.

How do you know if you have a faulty fuse?

You do not need to remove a fuse or use special devices to test it for damage. Look closely at the wire of the fuse.

Any discoloration on the fuse tube or break in the wire indicates a faulty fuse.

Can a fuse look good but be blown?

Some blown fuses may look okay and some may not even have clear tubes, so you can't see if something is wrong.