Understanding Circuit Breakers and MIL-STD-202 vibration

[BrianG]

Hi all,

I'm hoping someone here has practical experience of airborne circuit breaker specifications that claim to 'exceed MIL-STD-202 method 204 curve A, 10 to 500Hz'or others which state '10g from 10Hz to 500Hz'

If you know this MIL spec you will understand that between 5 Hz and 54Hz it shows a flat line double-amplituded displacement of 60 thou, changing to a constant slope at 10g from there up to about 2kHz. I am particularly looking for more information as to whether this can be true since if you project the 10g line backwards from the 54Hz breakpoint towards 10Hz you get about 1.5 inches displacement!

I'm ideally looking for a magnetic type of breaker rated at 3A to give fast tripping but these seem to adhere to the 60 thou line of the MIL spec. I need about 100 thou tolerance over the range 5-50 Hz and it's only the thermal types that claim better vibration tolerance in this low frequncy part of the profile, but these are quite slow to trip.

Any ideas?

 

[VEBill]

I don't have practical experience with that exact specification, but I looked the spec up (rev. G) and found the following:

"2.2.1 Amplitude. The specimens shall be subjected to a simple harmonic motion having an amplitude of either
0.06-inch double amplitude (maximum total excursion) or 10 gravity units (g peak), whichever is less."

http://snebulos.mit.edu/projects/reference/MIL-STD/MIL-STD-202G.pdf

See the pdf file, sheet 77 of 191.

The maximum amplitude of 0.06-inch seems to address the question you're asking about the calculated 1.5-inch.

 

[IRstuff]

I concur with VE1BLL that the amplitude requirements of 202 are limited to 0.06" double amplitude below 10Hz.

But, I'm unclear as to what you're asking. If a supplier claims that he exceeds 202 curve A at lower frequencies, then he is presumably exceeding the amplitude requirement.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[BrianG]

Thanks for the replies guys.
VE1BLL: Yes, I have this spec and I understand that the double-amplitude displacement is 0.06inch over that frequency range, whichof course is less than 10g. However, the various manufacturer's claims that 'exceeds the 202A curve', or in some cases 'withstands 10g from 10Hz to 2kHz' produces my query about the actual displacement under such test conditions.

IRstuff: I need to know by how much 'exceeds' means - if indeed it's actually possible, i.e. 1.5inch double amplitude? The helicopter specification I am designing for requires the equipment to withstand double amplitude of 0.1 inch over the span 5Hz to 54Hz and 10g thereafter, so the 10g straight line bit is OK, but it's the frequencies below that i.e. less than 54Hz that seem to be the killer.

 

[VEBill]

If I were in your shoes, I'd contact the circuit breaker vendors and establish one-on-one relationships with their engineering or engineering-sales persons. Give them the requirement as written and see if they can meet it.

If they can, then they can roll you a new dash number and bill you for the extra NRE. And they'll also provide all the proof of compliance and C of Cs you'll need later.

Depending on where you are in the project, you might want to go back to the helicopter PMO and ask them about their weird/unique vibration requirement. Most of the time, they'll know exactly what they're asking for. Only once in a while will it be a typo.

If you're designing a box for an existing platform, then a useful 'cheat' is to ask the platform OEM what CBs they used. But the vibration can vary with position, so that can be a deadend.