Fuses on the panel, or not

On Wednesday, October 16, 2013 12:42:06 PM UTC-7, JohnDeRosa wrote:
A couple of comments about circuit protection some of which has been covered before in other threads;



- Pete Purdie's comment above about undersized wiring is on target. Bigger is better (within reason)! If you saw smoke you probably do not have Tefzel wiring. Get some!

- Fuses are great as they are cheap, fast acting and have ZERO voltage drop across its terminals. Cons are that they are more difficult to replace in flight and somewhat fragile (glass type).

Capitalized ZERO, like you really mean absolutely zero, nothing, zilch, nada? Ah in a word. No. A fuse will *not* have ZERO voltage drop. A fuse relies on resistance in the fuse element causing heating and mechanical failure of the element. A typical fuse for a few amp applciation might have a voltage drop of ~100mV to ~200mV drop at the fuse rated current, and that depending on the fuse type and ratings). If there was zero resistance the fuse would never work. Since there is resistance there will be a voltage drop. Now that voltage drop may be a lot less than a similar spec circuit breaker, especially for low trip current applications. And in both cases is not a simple linear relationship, vendors typically provide voltage drop as a part of the spec sheet of their fuses and breaker products and fuse specs will include both that drop at rated current and a cold resistance value.

On Sunday, October 20, 2013 12:31:32 PM UTC-5, Darryl Ramm wrote:

Capitalized ZERO, like you really mean absolutely zero, nothing, zilch, nada?
Ah in a word. No. A fuse will *not* have ZERO voltage drop. A typical fuse
for a few amp application might have a voltage drop of ~100mV to ~200mV drop
at the fuse rated current, and that depending on the fuse type and ratings).

Darryl - Mea Culpa. Yep, you got me on that one. Maybe I will do better this time.

Both fuses and breakers have an internal resistance which translates into a voltage drop (which depends on how much current you are drawing). Lets talk about a 1A load for nice round numbers (YMMV).

Littlefuse 3AG fuse specifications (common glass tube);

Rating Internal Voltage Drop Voltage Drop
Resistance at rated Current at 1A
==== ========== ================ ============
1A 0.19 0.19 0.19
2A 0.07 0.14 0.07
3A 0.04 0.13 0.04
4A 0.03 0.12 0.03
5A 0.02 0.11 0.02

Klixon CT series breaker specifications;

Rating Internal Volts Drop Voltage drop
Resistance at rating current at 1A
==== ========== ================ ============
1A 1.10 1.10 1.10
2A 0.35 0.70 0.35
3A 0.13 0.40 0.13
4A 0.09 0.37 0.09
5A 0.07 0.35 0.07

Take aways;

- Fuses have significantly lower voltage drops than breakers by an average of 4x ... but it AIN'T ZERO!
- The higher the current circuit protection device rating, the less voltage drop.
- Covering your panel with low rated fuses or breakers for each and every instrument isn't a good idea.
- Circuit protection devices in series with another device compounds the problem. For example a master fuse sourcing individual device fuses. Each layer drops more voltage.
- As had been said over and over again in this and other RAS threads, and bears repeating, a single large value fuse at the battery is your single best overall safety bet.
- One last thing - Per most manufacturers breakers should not be used as on/off switches. The only one I have found that can be used as an on/off switch is the Tyco W30 and W31 series.

- John

Anyone have any experience with the ETA - 1110-F112-P1M1-5A – circuit breaker/switch? Jonker used these on the early JS-1. See: http://www.newark.com/eta/1110-f112-...-5a/dp/98K5389

Tuesday, October 22, 2013 4:00:27 PM UTC-4, JohnDeRosa wrote:
On Sunday, October 20, 2013 12:31:32 PM UTC-5, Darryl Ramm wrote:



Capitalized ZERO, like you really mean absolutely zero, nothing, zilch, nada?

Ah in a word. No. A fuse will *not* have ZERO voltage drop. A typical fuse

for a few amp application might have a voltage drop of ~100mV to ~200mV drop

at the fuse rated current, and that depending on the fuse type and ratings).



Darryl - Mea Culpa. Yep, you got me on that one. Maybe I will do better this time.



Both fuses and breakers have an internal resistance which translates into a voltage drop (which depends on how much current you are drawing). Lets talk about a 1A load for nice round numbers (YMMV).



Littlefuse 3AG fuse specifications (common glass tube);



Rating Internal Voltage Drop Voltage Drop

Resistance at rated Current at 1A

==== ========== ================ ============

1A 0.19 0.19 0.19

2A 0.07 0.14 0.07

3A 0.04 0.13 0.04

4A 0.03 0.12 0.03

5A 0.02 0.11 0.02



Klixon CT series breaker specifications;



Rating Internal Volts Drop Voltage drop

Resistance at rating current at 1A

==== ========== ================ ============

1A 1.10 1.10 1.10

2A 0.35 0.70 0.35

3A 0.13 0.40 0.13

4A 0.09 0.37 0.09

5A 0.07 0.35 0.07



Take aways;



- Fuses have significantly lower voltage drops than breakers by an average of 4x ... but it AIN'T ZERO!

- The higher the current circuit protection device rating, the less voltage drop.

- Covering your panel with low rated fuses or breakers for each and every instrument isn't a good idea.

- Circuit protection devices in series with another device compounds the problem. For example a master fuse sourcing individual device fuses. Each layer drops more voltage.

- As had been said over and over again in this and other RAS threads, and bears repeating, a single large value fuse at the battery is your single best overall safety bet.

- One last thing - Per most manufacturers breakers should not be used as on/off switches. The only one I have found that can be used as an on/off switch is the Tyco W30 and W31 series.



- John

Quote: "Anyone have any experience with the ETA - 1110-F112-P1M1-5A – circuit breaker/switch? Jonker used these on the early JS-1. See: http://www.newark.com/eta/1110-f112-p1m1-5a/circuit-breaker-thermal-250v-5a/dp/98K5389"

Quote: "One last thing - Per most manufacturers breakers should not be used as on/off switches. The only one I have found that can be used as an on/off switch is the Tyco W30 and W31 series."

MANY good comments in this entire thread. I don't like (glass) fuses. I do like Tyco W31 breaker(s)/switch(es) in my panel. See http://www.aircraftspruce.com/catalo...?clickkey=5950.

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