Battery Sizing Basics Article Addendum

In the “Voice Data Video Today” article, starting on page 30 of the September 2007 issue, the author would like to elaborate on some points for additional clarification. The piece offered tips for sizing battery protection devices and circuit conductors in telecom DC power systems.

Battery disconnect circuit breaker:

In a DC power system with multi-string batteries, a fault in any one of the strings will include the current contribution from the other parallel connected battery strings — apart from its own contribution and from other parallel sources like battery chargers. An estimation of the total fault current is therefore required to select the interrupting capacity rating of the battery circuit breaker. You must consider the external circuit resistance of the battery circuit (such as cables, bus bars, shunts, etc.) to realistically find the current fed by each string. In most cases, this would result in a considerable reduction of fault current as compared with the short-circuit current of the battery (per the battery data published by the manufacturer) and enable selection of a breaker with optimum interrupting rating, resulting in a cost-effective design.

The dedicated battery circuit breaker used at each string normally features only the magnetic trip without thermal overload protection.

Example No. 2:

Battery disconnect circuit breaker AIC rating:

The conservative approach is to take the short-circuit (SC) current published by the battery cell manufacturer for estimating the total fault current. Based on this value, each battery string will see a fault current equal to:

(40 × SC current of each string) + (1.5 × charger current)
= (4 × 14,750A) + (1.5 × 1,600A)
= 61,400A

Therefore, a DC circuit breaker rated 700A, 65VDC, 65,000 AIC is required.

Computation of battery short-circuit current with external circuit resistance:

Considering the battery circuit cable (neglecting resistance of other elements such as bus bars, shunt, etc.), and the contribution of parallel battery strings 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.

  1. 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.

  2. 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.

  3. 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.