Candlez

in a classified hazardous area? would 60KA(RMS) value for example be acceptable and according to which standard/code?

Generally it depends on the fault rating of the allowed equipment and cabling, after all the whole point of hazardous area protection is to limit the temperature rise and ignition source energy, not the fault current (which is many orders of magnitude higher than the ignition source energy limits).

What standards are you working to?

Mainly IEC Standards

As rightly said by Jack, it is not the magnitude and/or duration of the short circuit current but the withstand capability of the electrical equipments installed in the location, that matters the most. Let the fault current be 100kA. But, as long as you provide Cables/Switchgear/Bus Bars/Equipment, etc. which can tolerate this 100kA fault current for the worst scenario fault duration (co-ordinated trip time of the switchgear associated with the fault) - why should you worry?

But, yes! there is this cost angle to it. Perhaps, your fault current is 100kA and let's say the fault duration is 3 seconds, then the expenses incurred on equipments for tolerating this magnitude & duration could be exorbitantly high.

Now, there lies an engineer's balacing act. You have to choose between economy & safety. If economy is your worry, then you can think of ways to reduce the fault current and vice versa.

Your answer is to the point however :

Don't u think it may be difficult to obtain certified explosion proof distribution board having 100KA as standard from market?

Do you think maintenance electrician working on distribution board having comparatively high fault current shall be exposed to a higher risk if accidantly exposed to a fault?

Why don't we limit the fault current by considering transformers having higher impedance ratio ?

Your views on this are welcomed

"Why don't we limit the fault current by considering transformers having higher impedance ratio?"

Are you not worried about a higher voltage drop due to the higher transformer impedance - which is going to prevail throughout the normal operation of the plant, unlike the fault which is a rare event?

And, 100kA was just for example. Don't take it in the face value! I am not very sure about certification procedures for explosion proofing.

And, if 100kA fault current is risky, what about 50kA or for that matter even 1 kA - can your maintenance technician survive a 1kA fault?

My initial exposure to the need for explosion proof equipment was in (coal) mining, and then later in steel mills. (In both cases we had to deal with flammable / explosive dusts and gases.)

In both cases, we minimized what actually went into the area classified as hazardous / explosive. We took pains to locate equipment like switchboards (power distribution equipment) outside of the hazardous areas. (In mining, equipment that had to work at the coal face had to be "permissible" (the MSHA equivalent of "explosion proof". This included portable / trailing power cables, but I guess they weren't really explosion proof. I'd guess that they had special materials to be less flammable and perhaps self extinguishing, but we did have cable faults that resulted in arcs and such. (Which quickly tripped the breakers feeding that equipment.)

(Areas of a coal mine "outby" the working face were made less hazardous by things like circulation of large quantities of air (nominally fresh--i.e., drawn in from outside) and things like coating the surfaces of the mines with "rockdust" which is intended to keep coal dust from getting into suspension in the air in the event of an explosion.)

(Aside: Trick question: What color is the inside of a coal mine? (The trick is at several levels--if you are in an unlit area, with no cap lamp, it is black. Turn your cap lamp on and it is light gray due to the rock dust coating (hopefully) all surfaces.)

Anyway, you don't want to put power distribution equipment inside the hazardous area. You can avoid this in a variety of ways, perhaps putting such equipment outside or in a different building, or providing sufficient ventilation in areas of the same building to be able to declare some areas as non-hazardous. (This must be done carefully, also, with provisions to avoid sparking from this equipment creating a hazard, and provisions to shut down the process if the ventilation fails, etc.)

Equipment that must go in the hazardous area (instrumentation, maybe, maybe some motors, although even those could be located outside the hazardous area via the use of long shafts, hydraulic transmission, etc.) must be suitable for the area. (I.e., explosion proof (or dust ignition proof for areas where the hazard is only a flammable / explosive dust).)

Power feeds to such equipment we always put in galvanized heavy wall conduit, and there are requirements to have approved gas and dust seals as conduit goes into such hazardous areas.

It sounds like you haven't done it yet, but early in this process, some interaction must occur between engineers very familiar with the process (and hazardous materials involved in the process) and the "authority having jurisdiction" to classify the areas. That is, to decide which specific areas of the plant have hazards that require explosion / dust ignition proof equipment, along with defining areas that have hazards but not quite as severe (i.e., Division I vs Division 2 areas). This is often done by the engineering firm responsible for the project, and then conducting a review with the "authority having jurisdiction".

The "authority having jurisdiction" is often the chief of the local fire department, who may involve others with more expertise in such subjects.

Needless to say, there is a lot of potential for liability in the event any of these decisions are wrong. Or, even if they are "right" but an explosion occurs.

Many thks for your comprehensive detail about Mines environments. This reminds me with the difficulties tecnicians may face when working also in oil refinery or gas plants where flammable gases/liquid/dust are handled.

Be careful if still there

You're welcome!

Re: Be careful if still there

Thanks! I'm not, not sure whether that's good or bad. ;-)

I should have mentioned one other thing. Early in my career, the company I worked for was self-insured, thus we only had to deal with the local "authority having jurisdiction".

Late in my career, we started using commercial insurance. At that point we had to start involving the insurance company in those discussions / decisions as well as the local authority.