Blow the fuse if threshold exceeded
Your interface Iclock exposes methods GetNow and GetUtcNow.
Your TestClock class implements it, but because you also want to be able to modify the values you end up by implementing the properties Now and UtcNow.

In my opinion would be a better alternative that IClock would expose a read-only properties,
and that your class would expose the public setter as well:

public interface IClock
{
    DateTime Now{ get; }
    DateTime UtcNow{ get; }
}

public class TestClock : IClock
{
    public DateTime Now { get; set; }
    public DateTime UtcNow { get; set; }
}


Your Increase method in Fuse class is hard to get grasp on.

So... I migth be wrong but you do not want to keep track of the current time when Increase method is called.
What this means is that this line Point = Clock.GetUtcNow(); would become

Point = Point ?? Clock.GetUtcNow();


Furthermore I would consider to move this to some other method.
It could be a wise choice to move it to the constructor, however if you want to have a better control about exactly which tim ite is related to you could have a start method.

public Fuse Start(){
    //if you want to you can check if Point is null and throw an exception if it's not
    Point = Clock.GetUtcNow();
}


It also seems that your Increase method sometimes decides to do his trick and call Reset method.
If I were a consumer of any API I would never guess that a method called Increase would reset something.

In my opinion one simple thing you can do it have a boolean flag that would decide if Reset should be called "automatically".

Obviously this change would be accompanied by a slightly better named method. 
Unfortunately nothing better than ShortenFuse comes to mind.

public class Fuse
{
    public Fuse(Threshold threshold)
    {
        Threshold = threshold;
        AutoReset = true;
    }

    public bool AutoReset{ get; set; }

    public Fuse ShortenFuse(int value)
    {
        if (TimedOut &amp;&amp; AutoReset) { Reset(); }
        Count += value;
        return this;
    }
    //...
}


One more thing. In your "tests" you do this:

(fuse.Clock as TestClock).UtcNow = new DateTime(2016, 11, 12, 9, 0, 0).AddSeconds(20);


Wouldn't it be much better to put the seconds on the constructor?