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#### EVDL List

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Discussion Starter · ·
I have a simple question about air gap and DC breakers. AC breakers don't need a lot of help
extinguishing an arc because the voltage crosses zero many times a second. But DC voltage is
constant and an arc might be sustained if the air gap is not large enough. And I assume that
higher voltage needs a larger air gap. And for the purposes of this question, let's ignore
magnetic blowouts, etc.

Suppose we have a breaker with gap X and we needed to increase it's voltage rating. 2 of these
breakers in series (mechanically tied so they trip together) would in theory give twice the air
gap, or 2X. But neither air gap is larger than the original air gap X.

My question is, does putting breakers in series increase the air gap (and voltage rating) by
adding them together? Or is the voltage rating about the same as the original since the individual
gaps are the same as the original? Don't forget, I'm asking so I can understand if putting
breakers in series increases their voltage rating. Or not.

Thanks

Dave Cover

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#### EVDL List

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Discussion Starter · ·
Hello Dave,

Series circuit breakers might work if you have a common trip type, like a
two pole or three pole circuit breaker, so all the poles will trip at the
same time.

With single breakers that are tie together with a tie handle, there is too
much movement between the two. Normally if a circuit goes into a over
current fault, than one breaker will open which then pull the next one with
a little lag time.

If this is the case, then one breaker is opening while the other one may be
close and you will have the same air gap.

There is also the frame size of the circuit breaker to be consider, not just
the gap between the contact pads. A 600 volt frame mechanism must also have
more spacing than a 120 volt frame or the increase voltage could take out or
shunt out the magnetic trips.

In our electrical work, we use industrial I-Line circuit breakers, which
plug in to a buss line. We can use these on AC or DC circuits. A circuit
breaker design for 3 hp load on AC is only good for 1 hp on DC. So it about
a 3 to 1 factor.

Roland

----- Original Message -----
From: "Dave Cover" <[email protected]>
To: <[email protected]>
Sent: Wednesday, September 12, 2007 5:49 PM
Subject: [EVDL] Clearing the air (gap), breaker theory question

> I have a simple question about air gap and DC breakers. AC breakers don't
> need a lot of help
> extinguishing an arc because the voltage crosses zero many times a second.
> But DC voltage is
> constant and an arc might be sustained if the air gap is not large enough.
> And I assume that
> higher voltage needs a larger air gap. And for the purposes of this
> question, let's ignore
> magnetic blowouts, etc.
>
> Suppose we have a breaker with gap X and we needed to increase it's
> voltage rating. 2 of these
> breakers in series (mechanically tied so they trip together) would in
> theory give twice the air
> gap, or 2X. But neither air gap is larger than the original air gap X.
>
> My question is, does putting breakers in series increase the air gap (and
> voltage rating) by
> adding them together? Or is the voltage rating about the same as the
> original since the individual
> gaps are the same as the original? Don't forget, I'm asking so I can
> understand if putting
> breakers in series increases their voltage rating. Or not.
>
> Thanks
>
> Dave Cover
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
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#### EVDL List

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Discussion Starter · ·
My experience with this is that you can add the voltage if you wire a
multi-pole relays in series. Last year I wanted to switch a 1500 watt
heater powered off my 156 VDC (nominal) pack, and tried using 3 pole, 28 VDC
rated relays wired in series. Just one 3P relay didn't work - one of the
contacts arced and the relay melted down in a very satisfactory smoky mess.
Two 3P relays wired in series (6 contacts in all) did the trick.

Dave Cover wrote:
>
> I have a simple question about air gap and DC breakers. AC breakers don't
> need a lot of help
> extinguishing an arc because the voltage crosses zero many times a second.
> But DC voltage is
> constant and an arc might be sustained if the air gap is not large enough.
> And I assume that
> higher voltage needs a larger air gap. And for the purposes of this
> question, let's ignore
> magnetic blowouts, etc.
>
> Suppose we have a breaker with gap X and we needed to increase it's
> voltage rating. 2 of these
> breakers in series (mechanically tied so they trip together) would in
> theory give twice the air
> gap, or 2X. But neither air gap is larger than the original air gap X.
>
> My question is, does putting breakers in series increase the air gap (and
> voltage rating) by
> adding them together? Or is the voltage rating about the same as the
> original since the individual
> gaps are the same as the original? Don't forget, I'm asking so I can
> understand if putting
> breakers in series increases their voltage rating. Or not.
>
> Thanks
>
> Dave Cover
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
>

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#### EVDL List

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Discussion Starter · ·
The breakdown voltage rating of each individual breaker will not change. But putting two of such breakers in series doubles the
voltage that you can safely break by splitting the voltage across each opening set of contacts. The big assumption here is that
they both open at the same time. As Roland mentioned if there is a lag or delay on the second trip, then the full overvoltage
will be seen across the set of contacts that open first.

To increase the voltage rating of an individual breaker you have to increase the surface area of the contacts. This allows
charge to spread more around the contact area and dilutes the charge density. Charge density to high (and gap to close) and you
get arcing. Conversely if you make the surface area smaller (imagine two sharp points) then you have to increase the distance
between them to prevent arcing. This generally doesn't help because to break a circuit the contacts are initially close together
and can lead an arc the whole time while opening. So generally increasing the surface area and to some extent the distance between
the contacts is how you increase the voltage rating of the breaker or contactor.

Mike,
Anchorage, Ak.

> -----Original Message-----
> From: [email protected] [mailto:[email protected]]On
> Behalf Of Dave Cover
> Sent: Wednesday, September 12, 2007 3:49 PM
> To: [email protected]
> Subject: [EVDL] Clearing the air (gap), breaker theory question
>
>
> I have a simple question about air gap and DC breakers. AC breakers don't need a lot of help
> extinguishing an arc because the voltage crosses zero many times a second. But DC voltage is
> constant and an arc might be sustained if the air gap is not large enough. And I assume that
> higher voltage needs a larger air gap. And for the purposes of this question, let's ignore
> magnetic blowouts, etc.
>
> Suppose we have a breaker with gap X and we needed to increase it's voltage rating. 2 of these
> breakers in series (mechanically tied so they trip together) would in theory give twice the air
> gap, or 2X. But neither air gap is larger than the original air gap X.
>
> My question is, does putting breakers in series increase the air gap (and voltage rating) by
> adding them together? Or is the voltage rating about the same as the original since the individual
> gaps are the same as the original? Don't forget, I'm asking so I can understand if putting
> breakers in series increases their voltage rating. Or not.
>
> Thanks
>
> Dave Cover
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

#### EVDL List

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Joined
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72,624 Posts
Discussion Starter · ·
This reminds me of the theory behind dual-point distributors (Did Imperials have those?). They had 2 point sets, so they could keep the gaps close (high dwell) without worrying about arcing.

David C. Wilker Jr.
USAF (RET)

---- Steve Condie <[email protected]> wrote:

My experience with this is that you can add the voltage if you wire a
multi-pole relays in series. Last year I wanted to switch a 1500 watt
heater powered off my 156 VDC (nominal) pack, and tried using 3 pole, 28 VDC
rated relays wired in series. Just one 3P relay didn't work - one of the
contacts arced and the relay melted down in a very satisfactory smoky mess.
Two 3P relays wired in series (6 contacts in all) did the trick.

Dave Cover wrote:
>
> I have a simple question about air gap and DC breakers. AC breakers don't
> need a lot of help
> extinguishing an arc because the voltage crosses zero many times a second.
> But DC voltage is
> constant and an arc might be sustained if the air gap is not large enough.
> And I assume that
> higher voltage needs a larger air gap. And for the purposes of this
> question, let's ignore
> magnetic blowouts, etc.
>
> Suppose we have a breaker with gap X and we needed to increase it's
> voltage rating. 2 of these
> breakers in series (mechanically tied so they trip together) would in
> theory give twice the air
> gap, or 2X. But neither air gap is larger than the original air gap X.
>
> My question is, does putting breakers in series increase the air gap (and
> voltage rating) by
> adding them together? Or is the voltage rating about the same as the
> original since the individual
> gaps are the same as the original? Don't forget, I'm asking so I can
> understand if putting
> breakers in series increases their voltage rating. Or not.
>
> Thanks
>
> Dave Cover
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
>

--
View this message in context: http://www.nabble.com/Clearing-the-air-%28gap%29%2C-breaker-theory-question-tf4432711s25542.html#a12648104
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
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