250.20 SYSTEM GROUNDING (B)

[mbrooke]

The 50% number was reached by assuming that the hot and EGC wire lengths and sizes were identical and that the only fault return path was to and through the grounded utility neutral. Normally the impedance of the CEE to POCO MGN path will be much higher than that of the neutral wire.
I did bring up the possibility of other voltage ratios later.

The 50% number is true for symmetrical impedance between the hot and the neutral (ie 20 volt drop on the hot would also mean a 20 volt drop the ground while the fault is happening)



However, I am questioning how that determination is reached on what the voltage will be like on the chassis relative to remote earth.


Anyone have any articles on what the values are intended to be? This has me really intrigued because on of the purposes (my understanding) is to keep voltage down while the fault is happening.

 

[GoldDigger]

Given that even 1/3 of the system voltage could be fatal, I think that for the bolted fault the only relief is opening time.
But for a less than bolted fault most of the voltage drop will be across the fault, which allows the EGC to hold the voltage to a safe level for the much longer opening time.
You have to consider both types of fault and protect against both.

 

[mbrooke]

Given that even 1/3 of the system voltage could be fatal, I think that for the bolted fault the only relief is opening time.
But for a less than bolted fault most of the voltage drop will be across the fault, which allows the EGC to hold the voltage to a safe level for the much longer opening time.
You have to consider both types of fault and protect against both.

You make a good point. One thing that I have always wondered is how our CMP came to determining EGC sizes as they play a critical role in disconnect times and reducing fault current.


In the IEC a lot of it is based on "earth fault loop impedance" where enough current must be drawn to fulfill set disconnect times. Studying up really gets you thinking about it, at least in my case.



In regards, could that be one reason why we use full size EGCs at the branch circuit level where the circuits tend to be the longest?

 

[GoldDigger]

...
In regards, could that be one reason why we use full size EGCs at the branch circuit level where the circuits tend to be the longest?

This is also part of the justification for upsizing the EGC when you upsize the hot(s) for VD reasons.

 

[mbrooke]

This is also part of the justification for upsizing the EGC when you upsize the hot(s) for VD reasons.

Good point.

Would anyone happen to know how the 50% number came about? I know Im stuck on it but its getting me to think about EGCs in news ways.

 

[bobby ocampo]

Good point.

Would anyone happen to know how the 50% number came about? I know Im stuck on it but its getting me to think about EGCs in news ways.

Size of EGC should be based on maximum short circuit fault. For solidly grounded single line to ground fault and double line to ground fault which ever is higher. For ungrounded prospective double line to ground fault. HRG prospective double line to ground fault too. The EGC should be large enough not to become a fuse for proper opening of OCPD.

 

[bobby ocampo]

Check IEEE 242 Buff Book

Chapter 9. Article 9.4.2 Conductor Temperature

 

[don_resqcapt19]

Size of EGC should be based on maximum short circuit fault. For solidly grounded single line to ground fault and double line to ground fault which ever is higher. For ungrounded prospective double line to ground fault. HRG prospective double line to ground fault too. The EGC should be large enough not to become a fuse for proper opening of OCPD.

Unless you have some very unusual power source, the EGCs as specified in 250.122 will not become fuses and will provide a low enough impedance to cause the OCPD to quickly open. For those very very rare cases where the conductor size specified in 250.122 would not be large enough 110.10 can be used to specify a larger EGG.

The withstand rating of copper conductors is actually quite high, for example a #14 copper conductor can withstand 1,000 amps for 3 cycles without damage. It can also carry 97 amps for 5 seconds without damage. The definition of damage for an insulated conductor is that that conductor temperature does not exceed 150?C.

 

[kwired]

How is this 50% number reached? A larger EGC would defiantly reduce the voltage rise. And, if I bonded the faulting appliance to a mat or UFER below my feet that voltage would go down even more. Even at 277 volts, my understanding is that when the fault does happen in a building that on average the voltage drop across the EGC will cause a voltage around (approximately) 1/3 to show up relative to remote earth.

How often is a person in contact with remote earth? Probably part of reason we don't normally see electrocutions in these incidents, the voltage of nearby grounded/bonded objects rises the same amount as the EGC that has the fault imposed on it, the nearby person though they may for a very brief period be above true earth potential never get shocked unless they are also in contact with true earth potential. And any bonded objects that contact earth probably leak some current via earth paths but would create voltage zones in the earth around those objects.

 

[mbrooke]

How often is a person in contact with remote earth? Probably part of reason we don't normally see electrocutions in these incidents, the voltage of nearby grounded/bonded objects rises the same amount as the EGC that has the fault imposed on it, the nearby person though they may for a very brief period be above true earth potential never get shocked unless they are also in contact with true earth potential. And any bonded objects that contact earth probably leak some current via earth paths but would create voltage zones in the earth around those objects.

Very true, but keep in mind the case of a building without a UFER bond. In such a case the person is theoretically in contact with remote earth?

In commercial though where conduit and steal framing is used so many paths exist back to the source that it makes little difference in voltage rise.