Adjustable fuse that can be reset

 Those of you who have read my previous posts on safety on the bench, will be familiar with the article I wrote on safety when working on tube radio repair.

My home wiring system has a 10A resettable fuse on the sockets in my shack. That is 2,300 Watts of power! 

However, with all my test equipment running, I barely use 2.5A. So I wanted a way to adjust how much power is delivered to my bench using an adjustable fuse that can be reset.

Most people will think, why do I need this as I have FI protection on my circuit that will detect a ground fault quickly? Well, if you read the article in more detail you will see that a part of my bench is not referenced to ground and for a very good reason. In a case like that, the FI will not see a fault. So the dim bulb gives protection but a resettable fuse is the ultimate current limiter.

I also committ the ultimate no no, which is connecting extension cords in series. This can easily lead to an overload situation. However, I do this because I need the length and the large number of low power devices spread all over my bench. I also want the added security that I do not overload the first extension cord in the link.

So, I came up with the following concept to limit power and make it adjustable:

All components are from F&F.

• Controller EPP 620
• Current Transformer TI-30/5
• Relay PP-1P

The controller is set to mode C:

I use Relay R1. When Imax is reached, R1 latches and connects terminal 7 to terminal 8 (NO), effectively cutting off current to the rest of the circuit (default mode is terminal 7 to terminal 9 = NC). It can not be reset without turning the power off and on again, which in my case means, unplugging it and plugging it back in. So it is fail-safe.

The current sensing transformer is connected to terminals 1 and 2.

The EPP 620 can switch a maximum of 2 x 8A. So to avoid overload, a relay PP-1P was added, which can switch 16A. More than enough for this application as the main board breaker trips at 10A.

The setup was tested with different loads and works exceptionally fast, tripping very, very quickly. The adjustment is very sensitive and can be set as low as 800mA.