gs into the battery string at fault, a rough calculation reveals that the total fault current would drop down to 49,000A (approximately), including the contribution due to battery chargers.
Nominal circuit voltage of 48VDC, battery internal resistance of 0.136 milliohm per cell, 400kcmil cable DC resistance of 0.033 ohms per 1,000 feet (per NEC Table-9), and the circuit cable length of 50 feet (for each battery string) are considered in the computation.
Therefore, a DC circuit breaker having AIC rating of 50kA, instead of 65kA AIC, would be adequate. This breaker would be less expensive in meeting the requirement.
The battery circuit cables (two parallel runs of 400kcmil) are suitable to withstand the computed short-circuit current.
— Shanmuga Pandian,project manager, SOFCON,
Al-Khobar, Saudi Arabia
Forensic Casebook Feedback
I have two comments and one question concerning the “Case of the Knockoff Motor” article, starting on page 22 of the September 2007 issue.
At the top of page 27, the author states, “the use of a green insulated conductor as a phase conductor could have fooled even an experienced electrician.” I take this as a slam on licensed electricians. An experienced electrician would never be fooled by an example like this.
At the bottom of page 27, the author states, “the use of a licensed installer would not have prevented this death.” I completely disagree, and take this as a second slam on licensed electricians.
The brief credentials of the author state he is a certified electrical contractor in Florida. I would like to know if he is a licensed electrician. For example, in Michigan, contractor licenses can be under another's master license.
— Pat Wiitala, master electrician,
Bay Electric, Hancock, Mich.
Author's reply: 1) The documentation was in Chinese. Electricians who can read the Chinese language seem to be in fairly short supply in Florida… But seriously, I was talking about the original installation of the ultraviolet coater. I spoke with several electricians that I have known for most of my career, all who said they do not normally check the internal wiring of equipment they install unless they're specifically asked (and paid) to do so. My employees certainly never did. I am not an attorney, but I would hazard a guess that such inspections may even be inappropriate without the owner's express approval.
2) One of my points was that the green conductor was never checked to see if it was, in fact, grounded before using it as such. The several electricians I queried replied that they seldom verify that the green conductor is grounded unless some unusual circumstance dictates it. A couple of cases like this have helped convince me to carry an electric field detector “pen” and to use it, even on green conductors. I suggest the same to everyone who works around electricity, especially amateurs.
3) You are correct. The state license in Florida is a license to be an electrical contractor. Prior to the Florida state licensing law, local jurisdictions issued (and still issue) certificates of competency. I received my first such certificate from Hillsborough County in 1972.
— Curtis Falany, FORCON
International Corp., Brandon, Fla.