How do you calculate microhms from millivolt drop test?

[STXERS17]

I've seen this exact question. They are basically testing you on two fronts:

1.) Can you calculate the value using ohms law?
2.) Based on your experience, is this acceptable?

250 micro-ohms is way to high for a 1600A breaker of any type. Test enough breakers and you will understand.

You can usually find recommended values in circuit breaker instruction books, and if you compare them, you will find that they are all pretty similar. Here is an example from Square D:

Attachment 195

NW08 = 800A, NW12 = 1200A, etc.

Great info, thanks.

[mattj77]

15. You have performed a millivolt drop test on a 1600-ampere, low-voltage circuit breaker. The result is 25 millivolts at 100 amperes. What is the contact resistance, in micro-ohms, and is the value acceptable?
Your Answer: 400 microhms; no
Correct Answer: 250 microhms; no

I'm not sure why my answer is wrong.

[speark13]

15. You have performed a millivolt drop test on a 1600-ampere, low-voltage circuit breaker. The result is 25 millivolts at 100 amperes. What is the contact resistance, in micro-ohms, and is the value acceptable?
Your Answer: 400 microhms; no
Correct Answer: 250 microhms; no

I'm not sure why my answer is wrong.

You are applying Ohm's law incorrectly.
V/R=A
Solving for R
V=R*A
V/A=R
.025V/100A=.00025 ohms