I hope discussions such as this will not be censored. I will prepare illustration
Please check my post with illustration above. Please be patient as I want to explain this step by step. I want to be proven wrong based on IEEE 80. We do this in preventing electrocution in substations. Thanks for your patience. Hope there will be no censorship.
I don't know what you mean by censorship. Just asking questions about the national electrical code or electricity in general is not going to result in any issues.
I have been censored in my other post. Its not you.
Yes you are correct. Thanks. My follow up question is what if the DELTA IS UNGROUNDED, Will the person touching the metal panel be electrocuted assuming the line to line voltage is 240 volts? 480 volts? Without EGC.
How low will the voltage be to ground in your experience will it be specially if the panel is close to the transformer?
I wil show the principles later to prove my point or I will be proven wrong. Physics of electricity is the same regardless of the voltage. Thanks for your comment. Let's continue to challenge the status quo.
Not necessarily proven based on the available evidence that no one is questioning.
The question is will the person touching the metal panel be electrocuted if the system is UNGROUNDED?
1. Thanks. Is the approaching to zero volts because of the connection to earth of one of the current carrying conductor to ground?
Thanks. If the distance from the transformer is very close, will it very close to zero?
Now. Based on this illustration only. Connection to earth clearly reduces the touch potential if in this illustration line 1 is connected to earth. Let's call the top most current carrying conductor as line 1.
I am not sure what point you are trying to make. In the illustration shown the voltage from the grounded conductor to ground is low. That is more of a fact than a conclusion.
So lets repeat. FOR THIS ILLUSTRATION, Connecting to earth one of the current carrying conductor will bring the potential to zero or approaching zero voltage. Therefore electrocution is minimize provided that the voltage drop is not to large. Is this safe to say now again for this illustration?
Thanks for your comment. Regardless if there is a load or not the voltage to ground of the grounded current carrying conductor touching the panel will be approaching zero. The assumption is the metal panel is near the transformer so less voltage drop.
I think you answered it.
www.cirris.com
At low voltages, those below 600V, the earth is considered a poor conductor and therefore incapable of conducting enough current to reliably operate an OCPD as small as 15A.
Corner grounded is normally in industrial and commercial applications. In residential, supplied transformer by the utility is normally one leg with center tap delta transformer. Small residential is normally single phase 2 wire or single phase 3 wire. Thanks for your answer.
Please back read and see the first illustration exhibit that we are discussing. I will answer your question in different illustration later. I prefer to discuss it one situation at a time to prove my point on EGC and CONNECTION TO EARTH.Not enough detail but…
Typically when an OCPD fails you have to consider zone 2 or backup protection. So for example in a typical residential panel normally the branch circuit breaker operates. If it doesn’t then the main will. However how fast this happens depends on the current. Say available short circuit current is 2 kA. Most residential breakers are UL curve C so they trip at 6-10x the marked current within about 1-2 cycles. So the 15-20 A breaker easily trips. But on a 400 A main it won’t trip instantaneously. We have to look at the thermal trip curve. We are at 2000 /400 A = 5x so we should trip in 3 seconds.
If it was a little higher, even 2.5 kA would trip “instantaneously”.
If it is only say 1 kA then we are at 2.5x and it can take a long time, about 20 seconds to trip. By then depending on the fault, it might melt/burn itself out. That’s the problem with high resistance faults.
In the mean time we have contact with a grounded (bonded) surface. Typically equipment grounds have lower impedance than the neutral. But say the hot, neutral, and ground are all similar. In my example with 2 kA on a 120 V circuit we know the impedance is 0.06 ohms assuming little voltage drop. As per this web site:
Wire Resistance Calculator - Cirris Inc
House wiring using 14 gauge is about a quarter ohm per 100 feet. Even if the bonding is near zero we would still be no more than about 20 feet from the distribution panel. If we are15 feet away we get 0.025 x 1.5 = 0.0375 ohms drop. We get the same drop in the neutral so to achieve 0.06 ohms with a parallel fault path the impedance must be0.01 ohms (parallel resistors). So we have two resistors in series. So the resistance on the neutral/ground circuit is 0.06 -p.0375 = 0.0226 ohms so by using the ratio the voltage at the fault is 120 x (0.0226 / 0.06) = 45 V.
As far as “electrocuted” you can be shocked at any voltage. You may not feel it but current flow is there. So technically yes.
Human resistance is about 1,000 ohms worst case through the epidermis so it does not affect the fault but does result in 45 / 1000 = 45 mA of current. This is not fatal but it will hurt and a victim can’t let go. Most safety standards set an upper limit in safe voltage at 50 VAC.
This is on a good, maintained system. On a poor system the fault current goes down so trip time increases. Voltage drop is not nearly as good so trip times are much longer and the shock hazard greatly increases.my example though is pretty “typical” for residential cases.
