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