I have a 1986 300 SDL Mercedes and I am having some problems with the glow plug relay box. I have been working on the car re-assembling the intake manifold and the injector lines. While working on the car, I have had the battery connected and disconnected. I have been trying to start the car and keep blow the fuse on the relay box a couple of times. My question is what causes the metal fuse bar to blow on the relay box? Does it have anything to do with the battery cables not being tight and creating some kind of a short when attempting to glow the plug? That is the moment when the fuse blows. Is there something else that I should giving my attention?

During the process of assembling the parts mentioned above, I have blown the fuse three times. This just puzzles me and if anyone could offer any suggestions it would be helpful.
The car does have 246,000 mile on it.

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