A

apatti

Well Known Member
All,

If a double pole switch is rated for 15 amps, is that 15 amps per pole? So, if my application requries more than 15 amps, I could use a double pole switch and get up to 30 amps. Is that correct?

Thanks,
 
The short answer is No.

The slighting longer answer is that combining poles will make the switch good for more than 15A but less that 30A, but to determine that number is more trouble it is worth.

The full (probably more than you want to know) story is:

Most likely the rating is 15A per pole. However, if you connect both poles together (in parallel), the current will not split exactly between each half of the switch. So if you put 30A through the switch, you might have 16A through one pole and 14A through another - that would put you outside the switch ratings. So from a practical standpoint, you don't have a 30A switch.

You could do some testing to determine the difference in contact resistance and come up with combined-pole rating that would be valid for this particular switch at this particular time. However you would need to periodically remove the switch and retest to ensure the current was splitting in the same way. Possible to do this, but it is much less hassle and money to buy a 30A switch.

Now if this is just a bar bet with your buddies that you can put 30A though this switch a few times without blowing it up - you'll win that bet.

Is this switch going in an RV? 30A is a lot of current for anything controlled directly by a switch in a RV.
 
I should have included in the original post that I have the electric bible and have read the chapter on switches several times. I did not find the answer to the question I posted.

Bharral's response makes sense. To the extent that there is variation in resistance between the two sets of contacts, you don't get the full doubling effect. Also, if one set of the contacts fails, you won't necessairly know until you pull too much current through the other set.

The application is the E-buss direct switch. I have placed seldom used items like the landing/taxi light, flaps, trim, and fuel pump on the E-buss. With everything on (and transmitting) the current can be in the mid 20's. I would rather not use another contactor. So, I was hoping to just use a switch.

From the electric bible I understand that most switch damage occurs during operation of the switch. In this case, I plan to turn on the E-buss just after the master. Most of the loads will be getting power from the main buss at that time. Thus, when switching on the E-buss, there should be easily less than 10A being pulled through that switch. However, when turning on non-resistive loads there will be an in-rush current. Does anyone know if the in-rush issue is as big a deal on a switch that is already "ON"? Or is it mainly just an issue during the "make" of the switch? The reading of the electric bible makes me think it is only a problem during switch operation.

Thanks again,
 
For this calculation...

apatti said:
......
The application is the E-buss direct switch. I have placed seldom used items like the landing/taxi light, flaps, trim, and fuel pump on the E-buss. With everything on (and transmitting) the current can be in the mid 20's. I would rather not use another contactor. So, I was hoping to just use a switch.
.....,
Click to expand...

...I think it would be reasonable to assume that everything is NOT transmitting when the switch is activated.

Would you really flick this E-bus switch and hold down the PTT at the same time?

Since a comm radio is probably over 5 amps in Tx mode, does this change your calculations and let you use a more reasonably rated switch?
 
Tony,
Read the section of Bob's book on relays. You should never be putting more than 10 amps through a switch. Wire a relay between the power source and the bus. Wire the switch to energize the relay.
The switch will then only carry the current required to energize the magnetic coil in the relay. (1 or 2 amps at most) The relay (which is designed to switch heavy loads) will handle the switching of your bus.
What you describe is not an E (endurance) bus, as defined by Bob (and everyone who has read his book). An E bus handles those items that need to stay on during alternator out (endurance) operations.
Charlie Kuss
 
az_gila said:
...I think it would be reasonable to assume that everything is NOT transmitting when the switch is activated.

Would you really flick this E-bus switch and hold down the PTT at the same time?

Since a comm radio is probably over 5 amps in Tx mode, does this change your calculations and let you use a more reasonably rated switch?
Click to expand...

Gil,
Exactly! I expect I will NOT be transmitting or even switching on landing and taxi lights at the same time as I am operating the E-buss direct switch. In fact, I suspect the current draw during operation of the E-buss switch will be way less than even 10A. However, in the landing config (lights on, fuel pump on, flaps, transimtting, etc), the current draw could jump up to mid 20's for a relatively short period of time.

Charlie,
The assumption in Bob's book is that the switch will be "switching" the entire load. As stated above, I will not be doing that. Thus, my question to the group.

Past threads on endurance vs emergency busses have debated the merits/demerits of each. After I read both sides of the debate, I decided that I will place seldom used items on my E-buss so that IF I arrive at my destination (or suitable landing site) with battery power still available, I can lower flaps, turn on the fuel pump, perhaps even turn on lighting if I want. If I am low on battery power, I won't. But, I have the option. And, yes, if it is in fact just an alternator failure, you can turn the master back on and still have access to main buss items. However, if the problem is a main buss fault of some sort, you have to troubleshoot the problem before re-engaging the master. I would prefer not to have to troubleshoot in the air, if possible.
 
apatti said:
Gil,
snipped
Charlie,
The assumption in Bob's book is that the switch will be "switching" the entire load. As stated above, I will not be doing that. Thus, my question to the group.

Past threads on endurance vs emergency busses have debated the merits/demerits of each. After I read both sides of the debate, I decided that I will place seldom used items on my E-buss so that IF I arrive at my destination (or suitable landing site) with battery power still available, I can lower flaps, turn on the fuel pump, perhaps even turn on lighting if I want. If I am low on battery power, I won't. But, I have the option. And, yes, if it is in fact just an alternator failure, you can turn the master back on and still have access to main buss items. However, if the problem is a main buss fault of some sort, you have to troubleshoot the problem before re-engaging the master. I would prefer not to have to troubleshoot in the air, if possible.
Click to expand...

The logic of using an E (aka endurance or minimal load) bus is to ease pilot workload. Instead of having to shed all unnecessary loads one by one (flipping each switch or breaker), you can simply flip the E buss switch on (Bob recommends no more than 7 amps on this bus) and switch the master switch off. In this manner, the pilot can quickly shed excess electrical load if the alternator fails (assuming a one alternator ship). He is then free to deal with any other issues, rather than being distracted by electrical problems.
In this manner you give yourself the best chance of still having enough left in the battery to use the items needed during landing.
Items such as the electric fuel pump and electronic ignition are best sourced off of the battery bus. In that manner, they will always be available if/when needed. Prior to the landing phase, you can shut down items on the main bus that won't be needed for landing. Once this is done, the pilot simply turns the master switch on just before starting the landing phase, which brings the main bus back on line. The landing lights, position lights and other items needed to land can then be turned on for the last few minutes of the flight, as needed.
Charlie
PS Please check your private messages
 
Hmmm....

apatti said:
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The application is the E-buss direct switch. I have placed seldom used items like the landing/taxi light, flaps, trim, and fuel pump on the E-buss. With everything on (and transmitting) the current can be in the mid 20's. I would rather not use another contactor. So, I was hoping to just use a switch.
----
Thanks again,
Click to expand...
Why do you have the lights, flaps, and fuel pump on the E-buss? If you follow Bob N's philosophy, those items would remain on the main bus, which can be re-activated if you need them. The E-bus is for the items needed for safe continued operation under battery only conditions - with minimal load.
The time a switch's contacts receive the most damage is when you turn it off under load because breaking the contact is when the arcing damage is worst.
 
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