Choosing the right DC breaker to stay in the limits of battery bank

Dave Angelini · June 2015

BB. wrote: »

I thought was vtMaps did--That the arcing issue was only a problem for when the breakers trip... But reading a bit, I am not sure, but it appears the polarity of the current issues still even applies for "normal switching" too (such as if you manually turn off the breaker while the solar charge controller is charging).

I would do as vtMaps suggests and point the + terminal to the battery bank to protect against high current surge trips. And avoid turning off the breaker if there is charging current going through the wire (i.e., dark). In some cases, this is a good idea anyway... Turning off an alternator based charger (wind/water turbine, DC Alternator on a genset) when they are charging can cause problems (wind/water turbines can over speed, all alternators can over voltage and damage electronics directly connected to their output if the "load/battery" is switched off when they are at speed).

-Bill

I think this is the key to many problems! People will just flip the switch without removing loads. Take apart a typical square D type QO or QOU breaker and you can see the arc marks where a 2KW load was interrupted. In many cases the device (CC or inverter) can be shut down on the control panel and then the breaker can be shut down.

Johann · June 2015

I got a stupid question.
In many AC switching applications there are resisters and capacitors used across the switching point to avoid arcing.
Would there be a possibility that the same techniques can be used for DC?

This is a very interesting post and raises many questions and possibilities.

vtmaps · June 2015

Johann wrote: »

I got a stupid question.
In many AC switching applications there are resisters and capacitors used across the switching point to avoid arcing.
Would there be a possibility that the same techniques can be used for DC?

I think (but not sure) those techniques (in an AC circuit) are for interrupting inductive loads. Inductive loads, when interrupted can generate high voltages.

In DC systems it is common to use a diode to short out high reverse high voltage across a switch when interrupting an inductive load.

--vtMaps

Johann · June 2015

vtmaps wrote: »

I think (but not sure) those techniques (in an AC circuit) are for interrupting inductive loads. Inductive loads, when interrupted can generate high voltages.

In DC systems it is common to use a diode to short out high reverse high voltage across a switch when interrupting an inductive load.

--vtMaps

OK, in that case, could a diode be used to avoid arcing and burning breakers and contacts.

BB. · June 2015

Actually, for DC switching, the use of capacitors and diodes are very common to protect switches from arcing.

If you remember the old mechanical points systems for ignition systems. That used a condenser (capacitor) to protect the points from arcing until they were open far enough--Then the higher voltage would not be a problem.

https://en.wikipedia.org/?title=Ignition_system

And a diode can also be used to reduce arcing... Basically, the diode "shorts" the inductive load.

https://en.wikipedia.org/wiki/Flyback_diode

However, since the arcing is not really an issue with inductive loads causing voltage to increase beyond the "arc voltage" level (around 12-15 volts or so, depending on the material of the contacts), I don't think that a diode across the contacts/loads will really help when you are looking at native switching voltages >> 12 VDC.

Both Diodes and Capacitors can fail shorted--So placing them around the load or across the points of a circuit breaker just is adding to the number and types of failures and, possibly, making things less safe.

There are other ways of reducing arcing--One is to use a magnet to "blow out" the arc into the Arc Chute, but this makes the breaker "polarity sensitive". And other ways:

https://en.wikipedia.org/?title=Circuit_breaker

Low-voltage MCB (Miniature Circuit Breaker) uses air alone to extinguish the arc. These circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled down while the arc voltage is increased and serves as an additional impedance which limits the current through the circuit breaker. The current-carrying parts near the contacts provide easy deflection of the arc into the arc chutes by a magnetic force of a current path, although Magnetic blowout coils or permanent magnets could also deflect the arc into the arc chute (used on circuit breakers for higher ratings). The number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker.
In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.^[4]
Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF₆) to quench the stretched arc.
Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (less than 2–3 mm (0.079–0.118 in)). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 38,000 volts.
Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device. The maximum current value and let-through energy determine the quality of the circuit breakers.

There is quite a bit of science and engineering behind something as "simple" as circuit breakers and fuses.

-Bill

Dave Angelini · June 2015

Big believers here of Engineering but as a balance I quote the grey one.

There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
— Mark Twain
Life on the Mississippi (1883, 2000), 173
Mark Twain

OntheWay · June 2015

Ok, according to specs, the only "officially" hydraulic magnetic non polarized breaker in vicinity is outback PNL-XX-DC series. Asked 100A version to Wind & Sun, I hope they can supply me one or two.

inetdog · June 2015

The NEC does not really like "thermal only" breakers for most applications because they have problems interrupting a high amp short circuit fast enough to avoid downstream damage.
A magnetic (instant) trip at a much higher current than the nominal current of the thermal trip provides that additional protection.

A fuse is "thermal only" but there is no time delay for the transfer of heat from the heater to a bimetal element. The heater is the trip, allowing it to satisfy the "instant trip" (less than one cycle) function.

Choosing the right DC breaker to stay in the limits of battery bank

Comments

Categories