Terminal Contact Resistance

[bphgravity]

In Mike Holt's Grounding and Bonding book, there is an Author's Comment regarding to the sizing of the equipment grounding conductor. For those who have the book,it is on page 64.

It bascially states Table 250.122 may not be good enough if the available ground-fault current is so high that it effects the contact resistance of terminations. He then provides a table with breaker ratings and multipliers.

An example is also provided. After the calculation is made and the multiplier is used, the equipment grounding conductor is larger than required by Table 250.122.

Where did these multiplier values come from, and why is this not specified in the code?

 

[ryan_618]

Re: Terminal Contact Resistance

Bryan, I don't have that book yet, but I have done a little of my own tests on the values given in table 250.122, particluarly the note under it that refers you to 250.4(A)(5). It seems to me that the values given in 250.122 assume a 3-5% voltage drop. If you were to exceed this voltage drop on the conductors (thereby increasing the impedance of the circuit), the total amperage under line to gound fault are lower than 4-5 times the rating of the OCPD. Try it out. I used a VD calculator to speed up the process a bit.

Sorry if I threadjacked, but I figured this was somewhat relevant.

 

[ron]

Re: Terminal Contact Resistance

I don't know what is in the Holt Book, but there is a theory that contact resistance increases during a fault due to magnetic forces and heating. Which makes our short circuit calculations even more theoretical, since we do not account for this.