Why should you be careful when choosing the size of a relay contact or fuse?

[Ivan Seeking]

Over the years I have run across a few surprising facts about electrical components. I thought to pass on a couple of these [somewhat obscure?] interesting and often important details. So here are a couple of questions to chew on.

1). Why is it bad to oversize a relay contact? In other words, if I have a ten amp resistive circuit, why would it be a bad choice to use contacts rated for 30 amps to switch the load? Cost and size are not important.

2). Why is it bad to undersize a fuse - especially with high power applications? If I have a 200 amp circuit and I know that my load should never exceed 10 amps, why should I still use a 200 amp fuse in the main panel; perhaps with secondary protection added and properly sized for the 10 amp load?

 

[dlgoff]

1). Why is it bad to oversize a relay contact? In other words, if I have a ten amp resistive circuit, why would it be a bad choice to use contacts rated for 30 amps to switch the load? Cost and size are not important.

Larger contacts tend to bounce more causing more arcing (contact burn-out and noise)?

2). Why is it bad to undersize a fuse - especially with high power applications? If I have a 200 amp circuit and I know that my load should never exceed 10 amps, why should I still use a 200 amp fuse in the main panel; perhaps with secondary protection added and properly sized for the 10 amp load?

Your 200 amp main circuit might in the future include other smaller (10amp) loops?

 

[JasonJo]

it is bad to undersize a fuse because of current spikes, especially with ac current

 

[cider_drinker]

Using an appropriate fuse on local end equipment is advised in all circumstances.

Undersizing fuses at a distribution level can result in un-necessary outages. If a 100A fuse were to be used in a substation that normally supplies 80 amps to a number of houses that have 200A main fuses installed, and a problem occurs in one of the houses, then the 100A substation fuse will blow first, outing all the houses when only one of them needs to be disconnected. This problem is enhanced when dealing with the higher levels of the electricity distribution system. This is why a text may refer to 'undersizing' to be bad.

Never use an overrated fuse for a local application. E.G. for 10A equipment, use 15 or 20A fuses, NOT 200A. AC inrushes are usually absorbed in a standard fuse by thermal inertia, so don't worry about surges blowing your equipment. Note that large inrushes my weaken your fuse, but this is a risk worth taking as the consequences of using a higher rated fuse can be fatal to the equipment or to you!

 

[Ivan Seeking]

I almost forgot about this thread.

Fuses are designed to absorb the energy of catastrophic failures. Especially when dealing with high amp circuits, say a 100 - 400 amp circuits, in the event of a failure an undersized fuse can explode with lethal force. Properly sized fuses are designed to handle this energy without detonating. Portland General Electric offers a course in which a laboratory film shows induced high amp failures. The undersized fuses exploded like bombs. Also noteworthy, due to the tens or hundreds of thousands of amps that can flow in large circuits for one two cycles of power before the fail safes [fuses or Circuit breaker's] kick in, any loose power cables can repel each other with lethal force as well.

Relays should be appropriately sized to extend the life of the contacts. Obviously under rated contacts will burn up. Over rated contacts tend to oxidize thus increasing resistance to the circuit and eventually ruining the contacts. Not only are most relays designed to provide a slight wiping action upon contact, this to wipe the contact clean of the oxidation layer, a certain amount of arcing is needed to aid in this process. If the contacts are too large there is not enough arcing to clean the surfaces. The source of this information was an engineer who designs relays.

These were both big surprises which I why I thought to post these little factoids.