Are there any EE's on the board who feel comfy talking about short circuit calcs?

Factual Questions
1

We are being advised by an outside contractor to do certain things to comply with the new OHSA regulations regarding short circuit protection and arc flashes.
They made some suggestions, that if we followed, we could alter the protective gear and associated barriers needed at certain test locations.
One of the suggestions they made was to dial down the magnetic thermal trip sensitivity on some of the breakers from a 10 to a 7.
The question is;
How does adjusting the inrush current sensitivity of the breaker change the available short circuit energy at the test?
Another one of their suggestons was to change the rating of the breaker from a 65K aic to a 25K aic (correct me if I’m wrong, but the K aic rating of the breaker is the amperage that the breaker can withstand without exploding). Again, what does this have dick to do about the available arc flash energy at the test?

{{crickets}}

2

Changing the sensitivity of the breakers would not actually reduce the short circuit current.

What it may do, is to reduce the time for the breaker to operate, this means that the current carrying conductors are exposed to the short circuit current for less time.

This in turn then reduces the amont of heat build up in the cable.

There is a big disclaimer here, circuit breakers being mechanical devices will always take at least a certain time to operate, no matter how great the short circuit current, so you would need to check the manufacturers spec on the breakers first, if the circuit impedance is already such that it means the circuit breaker is already operating in such conditions, then changing for a more sensitive one will not do a great deal.

The idea is to calculate your nominal cable sizes, from this you can calculate your circuit impedance.

Once you have your circuit impedance you can then calculate what the short circuit current will be.

The idea is that your cable must withstand this current, without melting of exploding, until the protective device breaks the circuit.

When you look at specifying a supply circuit, you obtain your data from the manufacturers of the cable, this will tell you the impedance per yard/metre and it will also tell you the maximum power dissipation of the cable, both continuous and short term, usually as a collection of curves on a chart.(There are standardised charts in various EE handbooks)

If you are not able to obtain this data, or the operating conditions are unusual, such as a very warm environment, then you can calculate this yourself using the adiabatic equation.

S = sqr rt( I[sup]2[/sup] x t)/K

Where,

S = cable size in mm[sup]2[/sup]
I = r.m.s fault current in amps(not counting any limiting effects of the circuit protector)
t = disconnection time of the protective device(fuse, tripping breaker or whatever, given the amount of current flowing) - you probably need manufacturers data for this.
K = this depends upon the method of installation, and things like how the cable is affected by crowding in cable trunking etc, and its in this number you can include the ambient temperature (eg for copper cables, in PVC insulation, and bunched k=115) - there are charts available for this in the wiring regulations books.

The above comes with one particular assumption, that the circuit is disconnected rapidly, certainly less than 500mS as the temperature rise in the cable would change the circuit impedance and so change the short circuit current, and in practice far faster disconnection than this is almost always achieved.

You can rearrange the adiabatic formula to derive the unknown value you seek, see link.

http://www.tlc-direct.co.uk/Book/3.7.3.htm

in fact a reasonably simple and authorative explanation of what we are trying to achive is here, from the same source.

http://www.tlc-direct.co.uk/Book/3.7.1.htm

It is not unusual to have to change the protective device following modifications to a supply system, extra bits are often added on but to make it more economic, you can reduce your cable sizes sometimes if you put in a more sensitive breaker.

The limiting factor in cable sizes is usually not the actual current demand, but more often the requirements of short circuit handling, it often means you use a larger cable for this than would normally be expected for the normal operating conditions.
This is because, the larger the cable cross section, then the lower its impedance, this will increase the short circuit current, and trip the protective device faster.
Added to this, the larger cable will be able to withstand this higher current for longer as it takes longer to heat up.

A good example is perhaps where you have a circuit with rewirable fuses.
The time to disconnect these is much longer than a trip breaker, so if you had those fuses, you may well find that a change of protective device will allow you to extend or modify your distribution system and stay within the wiring codes.

I’m doing a lot of this from memory so my explanation might not be too clear, it must be over 15 years since I had to do most of this kind of stuff.

3

Thanks casdave, that’s a lot of info and I will have to digest that before I can comment further.

That is exactly what we thought (knew). That is why their recommendation doesn’t seem in line with our thinking. We have scheduled a meeting with the contractor and I will relay any information that I feel contradicts what you and I feel to be accurate.

4

But it would limit the total ‘short circuit energy’. Does the regulation pertain to that or to something more like power?

5

It might limit the total short circuit energy, but it depends upon the circuit impedance.

It really depends upon the system impedance, and the incoming supply impedance, the contractors may well be right, but I personally would require that they prove it with certified measurements, and I would use a company not related in any way to the contractor to do the testing and inspection.

If the circuit impedance is already low, and the conductor sizes large, such as might happen if the user were fairly near to the final output transformer, and a large industrial user then the impedance might easily be so low that the prospective short circuit current is such that the breaker cannot trip any faster.

This is due to the physical limitations of the device, it takes time to sense the overcurrent, and this is affected by the size of the current, however, the time taken from sensing to the contacts opening has an upper limit and this is due to the mass of the contacts and the strength of the release springs.

The regulations are not at all interested (the UK regulations anway) in the total energy, not in the direct sense of the word.
All the regulations stipulate is that the circuit can safely handle any worst case scenario until it has been disconnected.

If you can design a circuit that can run a short circuit of 2000MA and it can handle this current indefinately and safely then as far as the regulations go, thats fine - it might get rather expensive on the utility bill though.

Such a circuit would be completely impractical, it would require all the conductors to be of unfeasable dimensions, possibly all the way back to the EHT step down transformer or even further back.

This does lead to another requirement of the regulations and also of most utility contracts, the the users system is designed in such a way as to not adversely affect other users supply, dragging down a huge current would cause a volt drop in the local utility supply network this would affect other consumers.

The contractors appear to be assuming that the worst case scenario fault would occur at switch on of the system, so there would be a massive inrush current.

The idea of having circuit breakers that take a predetermined time to trip is that when you do bring online a large inductive load such as an induction furnace, or a highly capacitive one such as an electroplating plant, then the surge in current, particularly the out of phase component which sets up the magnetic fields and electrostatic fields will not cause unwanted tripping of the breaker, and after a short time these inrush currents reduce bringing the circuit back within the normal running conditions for the breaker.

The most likely fault secnario, or at least the one you would have design in mind, is a system that is running, and the short circuit develops during this time.

Increasing the sensitivity might end up causing nuisance trip outs, other specifications of trip unit might well perform the task better.