Choosing Cct Protection

[GordMay]

Blue Sea Systems Technical Brief:
Choosing Circuit Protection
You may want to rethink the process that you use to select circuit protection. It’s not as simple as referring to circuit protection standards tables. Blue Sea Systems' engineers identify new factors to consider when selecting circuit protection.
Click for more information ... http://www.bluesea.com/Article_detai..._ID=292&id=355

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[Rick]

The Bluesea Systems tech brief is an attempt to explain to non-technical people just how to choose a fuse or breaker rating without pointing out the "big picture". All circuit protection fuse and breaker ratings are based upon what is called sets of time-energy curves. The time-energy curves are sets of data taken under different ambient conditions and, therefore, no one curve is adequate to describe or predict the behavior of a protection device for all conditions.

Understanding a time-energy curve is based upon understanding the difference between power and energy. Power is an instantaneous value of Amps multiplied by Volts whereas energy is power multiplied by time. A time-energy curve is mathmatically the time integral of power (lease bare with me on this). If the input voltage is constant then the time integral result is Volts multiplied by current squared multiplied by the variable time and this is referred to as the "eye-squarted-tee" formula for predicting a protection device behavior whereas normally the voltage term is omitted much like Amp-hour ratings omit the battery voltage in describing a form of energy capacity for batteries.

So what does all this mean? It means that ANY fuse or breaker will operate between an almost zero time huge current blow-out value and the value of current for which it may be operated constantly without interruption. Therefore the Bluesea Systems statement: "A fuse or circuit breaker should not continuously carry more than 80% of its rating to avoid overheating of the circuit protector itself" is false. If it were true the breakers used in houses would be causing problems for everyone, for example.

Because these time-energy curves are not normally handy for reference (assuming that they are easily interpreted) fuses and breakers are classed according to how fast (or long) they operate before causing an interruption of current. The Blueseas Systems technical brief is too sophomoric and very misleading if not downright wrong regarding intelligent choices of fuses and breakers, especially if one needs to prevent "nuisance blowing" of a protection device. I can imagine a Bussman engineer cringing at reading this brief.

With the time-energy curve concept in mind you may realize that wire jacket material is similarly rated for various currents versus time and ambient temperature. Good quality jacket materials do not melt, as Bluesea Systems engineers generalize in the brief. Thermoplastic jacket material melts and this is only a small set of materials available for use in wiring and is not used in aircraft, for example, because of that.

For further edification please refer to the Cooper-Bussman data showing time-energy curves and additional data for fuses:
http://www.bussmann.co.uk/images/Com...8%20Curves.pdf

As you can see the curves show operation for some fuses to be stable by 10,000 seconds or less (a practical infinity in some, not all, cases) which is 2.77 hours, a long time. You may also correctly conclude that intelligent choices of a particular circuit protection device for a particular ambient condition, wire, and load is not trivial and that is why codes exist to help make this safer by limiting just what is legally allowed. Do not let the Blueseas System tech note lead you into believing that anyone can make an easy evaluation and design something which is proper and adequate under your conditions of operation.

[GordMay]

Disclaimer:
Rick is a highly educated, intelligent, and experienced engineer. I am none of those. Nonetheless, I believe his criticism is overstated, and partly erroneous in fact

Rick:
You’ve unnecessarily complicated the issue of fuse/breaker selection, without illuminating the issue.

As you’ve indicated, the Blue Sea tutorial is not a learned treatise, just a cautionary brief, cautioning that sizing circuit protection correctly for your boat is very likely more difficult than you think.

“I squared T” is current in amps squared, multiplied by time in seconds.
It is used as a comparison between fuses and/or breakers, to judge speed of operation. Any given fuse or breaker with a high I sq T would let through more energy and take longer to fully operate ('clear' or 'blow') than a fuse link with a lower I sq T. Simply put, a higher overcurrent will clear a circuit protector more quickly than will a lower overcurrent.

The liked Busman site displays “Time-Current” curves, as utilized in standard engineering practice, without reference to Voltage nor “Energy”. With over 38 years in the electrical industry, including 10 years as a technical rep’ with Westinghouse (at the time the foremost circuit breaker vendor), I’ve never heard of a time-energy curve. The NEC (article 427-4) and CEC specify that circuit breakers shall not be loaded to more than 80% of nameplate amperage, unless the Brkr is100% rated. This 80% de-rating is also standard engineering practice. The breakers utilized in residential construction are NOT loaded to more than 80% of nameplate (ie: 15A Fuse or Breaker is only loaded to 1500W/12.5A max.)

Though it’s not a complete engineering design manual, I’ve seen nothing misleading nor incorrect in the Blue Sea “Brief”.

Worthwhile references:

http://search.netscape.com/ns/boomfr...7E20030621.htm

http://www.ewh.ieee.org/soc/pses/Dow...rs/90v03n3.pdf