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Grounding Riddle No.1 – Earth fault in no earthed system
On the other hand, the primary earthing system was not installed very well and related standards recommendations weren't applied in the erection stage; because the related engineers thought that the primary earthing was not very important in” no earthed systems” and they could ignore its erection in some area.
Many years after start-up and operation of the plant , some cable insulations were damaged due to no suitable operation conditions. One day a short circuit happened between one old cable and related electromotor casing.
Unlike anybody imagined, a severe short circuit current passed through the electrical system and ground surface which could be very dangerous for anybody in the site area.


How can you explain the reason of that accident?


Author : Hamid - From: Iran
 
#1
Thu, September 13, 2007 - 10:43
On most switchgear ordered for 3 wire ungrounded systems, there is usually some type of ground fault indication or alarming. Ungrounded systems are popular in continuous process applications where a circuit ground and the subsequent breaker trip causes more of a problem or expense than can be tolerated. Think about a plastic moulding machine, if a heater band fails to the machine frame (which happens) and the entire heating system goes down it can cause the damage to the entire machine as the plastic cools and hardens where it cannot be extracted. If this happens on an ungrounded system, the result is a corner grounded system (through the fault location). The machine continues operation until it can be shut down properly and a repair can be made. 
Author : guest
 
#2
Thu, September 13, 2007 - 10:45

Sounds like there were two different ground faults. The first one may have been there a long time and if there is no ground fault detection system, no one would even know that one phase has a ground fault. When the second fault happened on a different phase, you really had a line to line fault.
Don 
Author : guest
 
#3
Thu, September 13, 2007 - 10:45
Thank you very much for your good answer.

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-earth voltage increase 1.73 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. 
Author : Hamid
 
#4
Thu, September 13, 2007 - 10:47

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
 
Author : guest
 
#5
Thu, September 13, 2007 - 10:48
Thank you very much for your good answer.

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 increase1.4 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. 
Author : Hamid
 
#6
Thu, September 13, 2007 - 10:49

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. 
Author : guest
 
#7
Thu, September 13, 2007 - 10:50
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. 
Author : Hamid
 
#8
Thu, September 13, 2007 - 10:51

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