Earth fault in no earthed system

In three words "Very bad engineering" .

Could you explain a little more (what country was this in). Are you saying that your site had both a bonded earth system for protecting equipment and people AND an isolated earth system for doing the same. Why on earth did they do this, is it even allowed in your local regulations! Was this extra-low voltage control wiring?

Indeed, that mentioned site is a small part of one great electrical site which according overall designing procedure should be equipped by a suitable primary earthing system.

BEST REGARDS

M.S.J

Its seems the the industry was designed by some really freaks, or what I think it was not designed at all. One fine sunny morning the thought and started the installation of electrics.

And its also amazing how this industry get approval and cleanrence from concerned local electricity authority.

A very illiterate man is even knowing that earthing (grounding) in electrical system is most important part, then how any qualified engineer can ignore this.

And in simple words its absurd to use word "Designed" for such project or industry.

Guest,

There are 2 purposes for grounding (earthing): 1) to minimize the voltage which can be placed on exposed metal parts of electric equipment so people are safe when working around it, and 2) to provide a low-resistance path for a fault's current to return to the source so the overcurrent protective devices can turn it off.

A non-grounded (non-earthed) system may be chosen for a couple reasons--the circuit's voltage between conductors is fairly high (such as over 600 volts), or you need the equipment to continue running even with a short to ground until it can be fixed (such as avoiding safety problems or equipment damage from an unexpected shutdown).

When you have a non-grounded system, good design also includes a way to tell if any of the circuit conductors has accidentally become grounded. This would be some type of alarm or an indicator light which is monitored (tested and checked often). Then the ground can be found and fixed. The theory here is that the first fault to ground will be fixed before a second one occurs.

In the situation you describe, I suspect that there was no way to detect a first fault to ground, so during the many years of this site's operation, a fairly low-resistance fault to ground occurred and was not detected or fixed. This probably did not put enough voltage on any piece of equipment for it to be felt during use. Then, when a different conductor's insulation failed so it shorted to ground, this acted the same as shorting the two conductors together. However, as you noted, the short was through the equipment of the plant, so dangerous voltages probably existed along the path between these two fault locations until the power was turned off.

Possible fixes? 1) Convert the entire system to a grounded system, if the voltage between any live conductor and ground would then be 300 volts or less (a 480 or 575 volt delta system would not be OK, but a 480/277 volt wye system would be OK, and a 575/330 volt wye may be OK). 2) Install equipment which monitors the voltages between each of the live conductors and ground, and gives you a visible or audible warning when one of the live conductors becomes grounded, and ensure that this equipment is checked very often--like twice a day at least).

It sounds like there was very little (if any) inspection or regulatory supervision of this site and other problems as well. I hope that injuries were few and deaths none.

Best wishes--John M.

Thank you very much for your good answer. Fortunately in opposite somebody you can focus to problem and you don't need any things for thinking abstaining.

In regard to this topic, I can add to your good answer as following:

As you know in non-grounded system, when one of three phase conductors connected to earth, the rms of other phases voltage increase 1.4 time, therefore simultaneously earth fault on other phases in other places is possible specially when insulation material of electrical equipments are weakened. So in non-earthed systems two phase short circuit fault via earth pass is possible.

I am unsure how to interpret your post. I thought that the RMS voltage between two phase conductors would be unchanged if one was grounded or none grounded. True, the peak voltage is 1.4 times the RMS voltage, but this is again unchanged if none or one is grounded.

If these conductors are in a conduit, and you ground one, the effect on the others is hard to predict. If the conductors originally were all "floating" at about the same voltage from ground, then the effect of grounding would be to increase the voltage stress to ground on the other two. But, since the RMS and peak voltages between these conductors is unchanged, I would expect no difference in the probability of an insulation failure occurring.

If you are looking at equipment terminations where the conductors are further apart, yes, the increased voltage to ground could cause another fault.

I continue to believe that two phases short circuiting to ground almost always happens in sequence, with the first fault to ground occurring a significant amount of time before the second. Of course, there are situations where a fault from one phase to ground (in a grounded system) produces enough heat to vaporize everything in the power distribution equipment, but does not draw enough current to trip the upstream circuit breaker.

Thank you very much for your nicety.

You are right, unfortunately I mistake in over voltage factor writing (1.73 time is correct), but according to symmetrical phases-diagram we can to estimate the amount of rms voltage of healthy phases obviously. In regard to star point floating voltage in unearthed system, when one of three phases is grounded, other phase-earth voltage increase 1.73 times related to normal condition and this phenomenon can be caused the new earth fault in other weak point of electrical system.

Yes, your concern is correct. I appreciate your contributions-JMM

As for your question, "How can you explain the reason of that accident?"

If this really needs to be explained, then it would be futile because the explanation would never be understood by the people asking for the explanation.

This is an "Engineering Oversight" of the First Magnitude!

If I were you, I would be looking for a different company to work for so that I might live long enough to see my children grow up.