Understanding Inrush Current and Why Fuses Fail During Testing
If you’ve ever wondered, “Why did my appliance tester’s 10-amp fuse blow when I was only testing a 10-amp appliance?” you’re not alone. Everything may appear to be within the correct operating limits—but the real cause usually comes down to one factor: inrush current.
What Is Inrush Current?
When an appliance is first powered on, it does not immediately draw its normal operating current. Many appliances briefly pull much higher current than their rated value.
Common loads that produce high inrush current include:
- Motors
- Compressors
- Transformers
- Heating elements
- Switch-mode power supplies
- Power tools and handheld equipment
During startup, these devices can draw three to five times their rated current, sometimes even more. This short-duration surge—lasting only milliseconds—is known as inrush current, and it is completely normal.
Why a 10A Appliance Can Blow a 10A Fuse
A 10-amp appliance can easily generate an inrush surge of:
- 30A
- 40A
- Or even higher
Even though the surge is extremely brief, it still travels through your appliance tester. If the tester uses a fast-acting 10A fuse, the sudden spike can exceed the fuse’s tolerance and cause it to blow instantly—even though the appliance’s running current is perfectly normal.
Fast-Acting vs Slow-Blow Fuses
To prevent nuisance fuse failures, many appliance testers use slow-blow (time-delay) fuses. These are designed to tolerate short, high-current surges caused by inrush.
However, heavy inductive loads or very high inrush spikes can still exceed the momentary capacity of even a time-delay fuse.
Does a Blown Fuse Mean Something Is Wrong?
No. In most cases:
- The appliance is not faulty.
- The tester is not overloaded during normal operation.
- You did not misuse the tester.
The fuse simply reacted to an inrush spike that exceeded its short-term rating. Of course, if your appliance tester blows a fuse under no-load conditions, it would suggest your tester requires a service or repair. Seized relays can sometimes lead to Active-Neutral shorts which will instantly blow the mains fuse.
Need a Tester That Handles Higher Inrush Current?
If you regularly test equipment with motors, compressors, or switch-mode power supplies, you may benefit from:
- Appliance testers with higher inrush tolerance
- Equipment designed for leakage testing on inductive or heavy loads
Contact us if you’d like to discuss appliance testers or current leakage testing equipment built to handle higher operating and inrush currents safely.
