protection against a utility company bolted fault 4

[stevemechanical]

some assistance, please, on basic electrical principles:

why does all the electrical equipment (say in a residential building) such as service switches and panelboards need to have a greater ampere interrupt capacity (AIC) than the utility company maximum symmetrical short-circuit current bolted fault rating?

why can't a service switch (which is on the line side of the panelboards) with a very high AIC be enough to handle the utility company bolted fault?

 

[dpc]

The problem is that you don't know which device is going to clear the fault. The "utility bolted fault" is just referring to the amount of current that the utility source can supply into your system when you have a fault somewhere. Just because there is a something upstream with a higher short circuit rating does not mean that it will open fast enough to protect the downstream device (unless it is a tested combination).

 

[stevemechanical]

dpc, thank you.
i'm finally understanding this. ok, so i need to change the circuit breakers in the main distribution panel. how do i prove this will clear the the bolted fault condition?

 

[dpc]

The circuit breakers will have an "interrupting rating" or short circuit rating. This value must be greater than the available short circuit current. For low voltage systems, this is generally given in symmetrical amps.

e.g. - the available short circuit current is 15,000 A, symmetrical and all of your breakers are rated for 22,000 A. This is acceptable. If you have one breaker rated for 10,000, that's a problem.

BTW, the term "bolted fault" is just referring to a fault with no impedance at the fault ( a zero ohm fault).

 

[Senselessticker]

The Bussmann SPD Electrical Protection Handbook has a good (and simple) example of how to calculate fault current at a given location. I believe you and google it and download it. However, I strongly recommend getting someone with a solid electrical background to perform the analysis.

The utility contribution is only how much "juice" the utility will "pour in" to the system under a fault condition. In general (and I do mean in general)...the farther the fault location is away from the utility (in feet of copper), the less the max fault value due to impedance of transformers, copper (or aluminium) wire, etc... Bear in mind....I just made a VERY simplied and general explaination. I've done calcs where utility contribution might be 2k, but way downstream the fault value might be less than a hundred amps. Basically means you can install cheaper protective equipment the farther downstream you go.

 

[waross]

The feeders to your sub-panels will add impedance which will lower the available fault current at the sub-panels. This must be calculated, it cannot be assumed.
respectfully

 

[cuky2000]

Ampere Interrupting Capacity (AIC) or interrupting rating is the maximum momentary current rated capacity of the individual protective device that is obtained by test or by approved calculations procedure.

This rating represent the largest potential short circuit current which can occur in the location of selected application imposing severe mechanical forces and thermal stresses in the breaker, panel, cables and other circuit components.

If the available short circuits at the location of the panel exceed the SC rating of the assembly, the entire installation is exposed to fire, smoke, explosion and other hazard conditions. See sample of possible hazard conditions

For additional information in this topic see the enclose bulleting.

http://www.bussmann.com/library/docs/spd05/spd05-s05-pp78-88.pdf

 

[itsmoked]

Nice pics cuky.

My Refrigeration buddy told me about an experience like your righthand picture. He was working a large control panel that ran a cooling tunnel in our local gummy bear factory. They control the temperature of the tunnel by staging refrigeration compressors. Eight six cylinder compressors. They bang another one on as demand increases. Well he was working away and 6 were on. Then 7 went on and he noticed some warmth down by his feet but didn't actually connect it with anything. Then the eighth one went on and an orange glow appeared and lit up his boots. He looked and the line feeding all the contactors was glowing. It had long ago vaporized its insulation.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com