Circuit Protection, Interrupt Ability, and ABYC

[sailingharry]

Gord,

That may actually help this make sense.

In the case of a breaker, exceeding the AIC can either 1) weld the contacts shut, defeating the purpose, or 2) break it so that it can't be re-used. In either case, the boater is in trouble -- either the current flows forever, or the CB has to be replaced before you can resume operations.

In the case of the fuse, exceeding the AIC can result in a ruptured/exploded fuse. The current stops, which is of course critical. Then, the boater must install a new fuse to resume operations -- same as normal! The only "extra" is that you have parts on the floor. Perhaps ABYC doesn't consider that significant enough to require AIC rated fuses. And while I am comfortable with that on the Nav-Station distribution fuse block, I'm still not sure that makes sense on a heavy duty fuse direct to the battery. Here's where ABYC doesn't make a lot of sense:

Let's assume you have the 1-both-2 switch, a pair of 600A dual-purpose batteries, and don't believe in fusing the starter circuit. A typical, if slightly unimposing, system. A fuse is still required within 7" of leaving the starter circuit, typically at the battery switch. Let's assume further that house loads are 100A. Finally, lets assume that due to distance (20ft), and drop (3%), the run to the house panel is #0 (see Blue Seas calculator at Blue Sea Systems). I think that you would be ABYC compliant if you used a 100A Maxi (these are the heavy duty car type blade fuses, some vendors go to 100A). The AIC of this fuse is 1000A, which is well within the current that a dead short could see in this example! (Note that this layout, while representative of parts of my boat, is not what I have or what I will be installing -- it's just an example).

An interesting side note is that the Blue Sea calculator applies the AIC requirements to fuses. If you use it, and indicate a short 1A load as a branch circuit on a very small 660CCA battery bank, they will tell you there are NO fuses that are acceptable for use -- yet they sell just such devices!

Harry

[CharlieJ]

A fuse is still required within 7" of leaving the starter circuit, typically at the battery switch.

Harry-Not so. The starting circuit is the only circuit on the vessel that is not required to be protected by an OCPD.

The question of an AIC requirement for fuses has been debated by the ABYC E-11 Project Technical Committee with no real resolution, primarily because of your paragraph 2 and 3 in Post #16. It will come up again next week. By the way, Blue Sea Systems' Chief Engineer is extremely capable and conservative hence the addition of AIC ratings on the fuses that they sell.

One of the fundamental tenets of the ABYC Standards is personnel safety which trumps most everything else. If a MaxiFuse is subjected to several thousands of amps, it can blow apart spraying molten, or nearly molten, shrapnel from the fuse and pieces of the fuse holder into the surrounding area. This failure can easily become a personnel safety issue.

Setting the fact that the ABYC Standards are currently mute on AIC ratings for fuses aside, there is another fundamental question regarding AIC ratings for DC systems. Table E-11 Table IV-A in the 2009-2010 Standards is where we select the required AIC ratings for main and branch circuit breakers. This table is entered with system voltage and battery cold cranking amps (CCA).

Consider a bank of 12 x 6 VDC golf cart batteries in a series parallel configuration has an enormous amount of energy that could be very destructive in a bolted fault. Golf cart batteries do not generally have a CCA rating. Therefore, it can be inferred that E-11 Table IV-A does not apply to this installation. Engineering best practice or industry best practice or common sense dictates using an OCPD (fuse or breaker) of at least 5,000 amps AIC, but this is not an E-11 requirement because we can't enter the governing table.

To further complicate the issue, thin plate pure lead (TPPL) AGM batteries have the capacity to supply a fault current much, much greater than 5,000 amps for a short period of time.

CCA is understood by anybody using Table IV-A. It is my opinion that a better way has to be developed to properly reflect the potential bolted fault energy available in the battery banks that we are installing on today's boats.

The ABYC standards are constantly evolving. E-11 and most of the more heavily used standards are reviewed, and changed, every two years by a committee made up of the USCG, industry organizations; e.g., NMMA, ABYC, equipment manufacturers, boat builders and industry professionals. The committee is balance and the standards that evolve are consensus standards...no one individual or group predominates the proceedings.

Hope this helps.

[GordMay]

Originally Posted by CharlieJ

... CCA is understood by anybody using Table IV-A. It is my opinion that a better way has to be developed to properly reflect the potential bolted fault energy available in the battery banks that we are installing on today's boats...

Indeed.

For the most part, ABYC Standards are presented as simplistic “rules of thumb”, based, in part, on sound engineering principles.

Unfortunately, many situations, such as interrupting capacities, do not lend themselves to simple tabular solutions. Instead, I.C.’s should be determined by reference to specific fault current calculations.

Unfortunately, this isn’t easy (see below).

See ➥ http://standards.ieee.org/colorbooks...VioletBook.pdf

And ➥ Short-Circuit Calculation Methods

And ➥ Beware of Simplistic Fault Current Calculations

[sailingharry]

CharlieJ

Again, as I've said before, you seem to really understand this stuff! Are you actually ON the technical committee,or just close to it? Passing the confusion reflected in this conversation on to them might be of value.

First, the fuse off the battery switch. Yes, the ONLY circuit that is exempt is the starting, but in the classic 1/both/2 system, both batteries are "starting" and therefore fusing is not required (but certainly optional and some believe essential -- I'm still undecided). As soon as you come off the switch heading toward the "house panel" you are no longer in a start circuit, and then it must be fused. This exact scenario is in some diagram I looked at, I think on Blue Sea's site. And the reason I used that example is I wanted an example load that was a "main" device but also a load that could be met by a lower grade fuse.

With regard to CCA on golf cart batteries, Trojan doesn't list them, which of course presents problems with the table. As I mentioned earlier, Lifeline sells a golf cart battery with similar specs, and they list around 1000A CCA, a number that is consistent with other indicators and probably a good assumption to use.

With regard to Blue Sea's conservative nature, I am confused. They sell a fair number of products that use glass and plastic low level fuses, which I think are a great product for that "nav station" example I keep mentioning. However, their calculator says that they should never be installed on a boat (at least any boat with more than 650CCA). That's a mixed message, for sure!

I understand "best practices" and "industry standards," and I'm certainly not advocating using that Maxi fuse! My concern, and what started this whole thing, is "the rule." The acceptable minimum standard that all work must meet. The chapter and verse that a surveyor will quote when he tells the insurance company there is a problem. And, frankly, as long as Xantrex and Balmar sell devices that look like they are intended to be connected directly to the battery with a little inline fuse, we are likely to have 1000A AIC fuses used as main fuses in many cases. I agree that a 18 gauge wire isn't likely to be able to pull the power necessary to blow apart a fuse, but again, what is the standard? My read of ABYC, at present, is "none."

Harry

[GordMay]

Fuses rated 1,000 AIC may be perfectly suitable for use in series rated* circuits, such as Harry’s “Nav’ Station Example”.

Series ratings are defined as:
A short-circuit interrupting (AIC) rating assigned to a combination of two or more over-current protective devices which are connected in series and in which the rating of the downstream device(s) in the combination is less than the series rating.

[hellosailor]

"both batteries are "starting" and therefore fusing is not required " Heck, even a 25 year old Honda has the sole battery, which is an SLI "starter" battery, fused. Typically the wire run to a starter motor itself is not fused, simply because starters take so much power. Then again, typically that was a fusible link wire, and those aren't used much any more because as they "fuse" they literally catch fire, which is not a good thing. Better to use a dedicated fuse that blows instead of combusting during "minor" overloads. (A small starter motor typically draws 1-1.5 kilowatts for five seconds or less, a midsize diesel starter easily twice that. Way more than the fuse/breaker would see at any other time.)

Relays typically have as little movement as possible on the contact arm, so the contacts may only be 1/16" or 1/8" apart, perhaps this is more in a power breaker--I've neer cracked one open to see. As against 1/2" to an inch or more across the body of a typical fuse, so you can see that it would take a physically MUCH larger arc to jump across a fuse. And when that happens, there is nothing to "weld" shut in a fuse, they've been literally blown up. In a breaker--the contacts can and will fuse, even if there's enough arc the contact arm can melt from heat and then there will be no space at all, at which point it can weld closed. Radically different set of conditions.

Somewhere on the web...one of the utility companies has posted photographs of what happens when lightning strikes a power substation. Now THAT jumps big arcs at high voltage, so as a result the "breakers" in substations can literally pull a contact two or three FEET apart. And when they weld shut--bad things happen.

[GordMay]

Originally Posted by hellosailor

... Relays typically have as little movement as possible on the contact arm, so the contacts may only be 1/16" or 1/8" apart, perhaps this is more in a power breaker--I've neer cracked one open to see. As against 1/2" to an inch or more across the body of a typical fuse, so you can see that it would take a physically MUCH larger arc to jump across a fuse...

Not exactly true. See the typical Breaker Construction:

[hellosailor]

There's sure a lot of "stuff" in there, Gord. It is hard to tell from the picture, just what is the scale or which parts actually break the circuit. Or how the "ARC extinguishers" function. Can you elaborate on any of that? And how a thermal breaker differs from a magnetic breaker internally?

[GordMay]

The switching contacts, on a typical 15A residential circuit breaker (as pictured) will be separated by about 1" when “tripped”. The breakers we use on boats will have a slightly smaller separation.

Arc-Chute design is very technical, and proprietary. Basically, the arc generated, when the contacts first begin to open, is diverted down the chute (rather than tracking the opening contact), thereby extinguishing the arc.

Magnetic Circuit Breakers use a three dimensional electromagnetic coil, whose pulling force increases with the current. The circuit breaker contacts are held closed by a latch, so that when the current in the coil goes beyond the rating of the circuit breaker, the coil's pull releases the latch which allows the contacts to open with a spring action.

Thermal Circuit Breakers employ heat to break the circuit current flow, and consist of a bimetallic strip, made of two types of materials welded together. At high heat levels, this strip bends at an angle (due to differential expansion) that pulls the circuit breaker's lever down, and breaks the connection between the circuit breaker's contact plate and the stationary contact plate.

Thermal-Magnetic Circuit Breakers employ both sensing & switching techniques, where the magnetic feature responds (instantaneously) to sudden current increases (short circuit), and the thermal feature responds (inverse time delay) to slow current increases (overload).

Blue Sea Systems “A” and “C” Series breakers, typically used on boats, are actually Magnetic-Hydraulic type.

See the tutorial ➥ Electrical Circuit Breakers

[hellosailor]

Thanks, Gord. That would seem to imply that the cheaper thermal type are indeed more likely to fuse from an overload (arcing/fusing before there is any heat buildup to pull them apart) compared to the magnetic ones.

[GordMay]

I don’t recall ever seeing a Thermal-Only device used in circuit protection. They’re normally used, after OCPDs as “overloads” in equipment protection schemes.

While infrequent, circuit breakers occasionally fail to trip, or fail to clear a fault.
The number one failure mode of all circuit breaker types* is opening when it should not (probably from a lack of selective coordination or improper trip setting), and, ironically, the second most common failure mode is failure while in service (not in opening or closing).

In my experience, it’s not unheard of, but rare, that a circuit breaker fails “closed” (fused or welded shut).

* but primarily on high voltage draw out metal-clad breakers.

BTW: It's been about 25 years since I did any applications engineering (I'm NOT an engineer), so, I'm certain we have Engineer members better equipped to answer these more sophisticated questions.

[hellosailor]

GE 20 Amp 1 In. Single Pole Circuit Breaker - THQL1120 at The Home Depot

Typical home-use circuit breaker. I've been told they sometimes wear out (i.e. then refuse to hold closed) and have to be replaced after maybe 20 years, because they are usually thermal breakers. Obsolete? Magnetic?

185-Series Thermal Circuit Breaker Panel Mount - Blue Sea Systems
Bluesea 185-series. Says "Thermal" right up front. AFAIK, much more common than the magnetic type, because they are much less expensive.

[GordMay]

I don’t understand your comment or question.

GE “THQL” breakers are ‘plug-in’ Thermal-Magnetic type, normally used in residential load centres.

BSS 185-Series are Thermal type, not normally used in marine (branch cct) distribution panels. These are similar to the “T1" breakers BSS recalled* a few years ago.

* Blue Sea Systems Recall Notice: Circuit breaker failed. User held circuit breaker in “on” position after it tripped due to either a short circuit or over current. Unit can then overheat, causing contacts to weld together. Normally the contacts will open but if an overcurrent occurs there is the potential for the contacts to remain welded and circuit breaker will be inoperative. This could result in an electrical fire. Blue sea is replacing all of these units with Bussman 185 circuit breakers.

[hellosailor]

"not normally used in marine (branch cct) distribution" maybe that's what I misunderstood, "branch" distribution. The breakers in a normal breaker panel are what I thought was "branch" distribution, and similarly, I've seen a number of those marked as thermal. (Most, of course, there's nothing to see or read since they're installed behind a panel with a rat's nest on the back.)

[GordMay]

Originally Posted by GordMay

I don’t recall ever seeing a Thermal-Only device used in circuit protection. They’re normally used, after OCPDs as “overloads” in equipment protection schemes...

Normally, Thermal-Magnetic (or Hydraulic-Magnetic) breakers are used in panels.