MIL-STD-202H.pdf - 第304页

MI L - S TD - 202 -3 1 2 4.1.2 Monit oring appar at u s . T he m oni t or i n g ap par at us s ha l l be c apab l e o f i n di c at i ng r e s i s tance s greater tha n a particula r value speci fied. D ur i ng e ac h c …

MIL-STD-202-312

METHOD 312

INTERMEDIATE CURRENT SWITCHING

1. SCOPE

1.1 Purpose. This test is conducted for the purpose of determining the electrical contact reliability of such items as

electromechanical relays, switches, etc., under intermediate current (formerly known as "minimum current") switching

conditions under which the contacts operate. An intermediate current switching circuit is one in which there is

insufficient voltage and stored energy to cause contact arcing during opening or closing of mating contacts, but which

have sufficient energy to cause melting of the contact material. Normal arcing of contacts at rated load levels often

act to burn off any oxide or other film on the contacts or provide localized melting at the point of contact, so that

contact resistance does not rise drastically. Without this arcing of the contacts, oxides and other contaminant films

can build up on contacts in component parts which have not been sealed adequately or which have contaminating

materials and vapors trapped within the enclosure due to improper manufacturing techniques. Such contacts will

develop unacceptably high contact resistance under intermediate current loads, unless the contact force and wipe are

sufficiently heavy to overcome any effect of contamination. Intermediate current switching is the range in which a

large percentage of loads occur. Therefore, it is extremely important that an intermediate current switching test be

imposed on all electromechanical relays and switches, which are to be used in this range. Relays and switches,

which pass both low level and full rated load tests, frequently fail when used in the intermediate current switching

range.

1.2 Precautions. Full rated load and low level life tests are not a substitute for the intermediate current switching

test. Successful testing at low level and full rated loads in no way reflects the capability of the relay or switch at

intermediate current loads. Statements or titles for component parts, such as "low level to full rated load" shall not be

used in specifications, unless intermediate current switching capability has been demonstrated by the requirement for

testing by this method.

CAUTION: A low-level run-in test is not equivalent to intermediate current testing and conversely intermediate

current capability does not indicate low level capability.

2. APPLICABLE DOCUMENTS

This section not applicable to this standard.

3. DEFINTIONS

This section not applicable to this standard.

4. GENERAL REQUIREMENTS

4.1. Apparatus.

4.1.1 Test circuit. Monitoring of the specified contact resistance of each pair of mating contacts shall be

accomplished on each cycle. Each contact shall be monitored on each closure. The apparatus, which cyclically

operates the contacts, shall be capable of automatically cycling the contacts at the rate specified. Resistive load

voltage shall be applied to the contacts and shall be 3.0 V dc to 10.0 V dc at 100 ±10.0 milliamperes (mA) such as by

means of a well regulated power supply which will provide the low voltage, controllable, and well defined voltage

source. Voltage, when required to energize coils in order to actuate the contacts, shall be as specified. Both

normally open and normally closed contacts of double-throw switching parts shall be tested. Multipole contacts shall

be connected with all normally open pairs of contacts loaded and all normally closed pairs of contacts loaded.

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MIL-STD-202-312

4.1.2 Monitoring apparatus. The monitoring apparatus shall be capable of indicating resistances greater than a

particular value specified. During each closure, the contact potential shall be monitored 10 milliseconds (ms) or more

after the end of specified contact bounce. The apparatus shall provide and record either manually or automatically,

the following information:

a. Number of contact closures with contact load applied.

b. If required, number of times contacts have performed as specified prior to failure to perform as

specified.

c. Sticking of contacts, when intended to be in the "open" condition, unless otherwise specified. Sticking of

contacts shall be defined as any failure of closed contacts to open as required during the cycling, or

indication across such contacts of less than 90 percent of the applied open-circuit contact voltage.

4.2 Procedure. Each pair of contacts shall be operated for 25,000 cycles (see note below) at the specified cycling

rate. The duty cycle shall be approximately 50 percent "on" and 50 percent "off". The component parts shall be

tested in a temperature chamber at the rated maximum ambient operating temperature with the required test load

(see 4.1.1). When specified, the final half of the test cycles shall be tested at room ambient temperature. Each pair

of contacts shall be individually monitored on each operation for failure-to-break (FTB) and for failure-to-make (FTM)

the test load, using the apparatus in 4.1.1 and 4.1.2. FTB shall be defined as a voltage drop across the contacts of

less than 90 percent of the applied voltage when the contacts are intended to be open. FTM shall be defined as a

voltage drop across the contacts greater than 0.1 times the maximum allowable contact resistance (in ohms), when

the contacts are intended to be closed. The voltage drop across the contacts shall be monitored for at least 50

percent of the time the contacts are closed and for at least 50 percent of the time the contacts are open, unless the

monitoring apparatus can be demonstrated to be capable of settling to a stable reading in a shorter period of time.

Any FTB or FTM shall either be recorded or shall automatically stop the actuating apparatus.

NOTE: Because the test is conducted for only 25,000 operations, it must not be inferred that the relays or

switches, so tested, are suitable for only 25,000 operations in the intermediate current range. Quite the contrary, if

the 25,000 operations test is passed satisfactorily, the relays or switches can be expected to be capable of switching

intermediate current loads well beyond the full rated load life cycles specified.

5. DETAILED REQUIREMENTS

5.1. Summary. The following details are to be specified in the individual specification:

a. Maximum contact resistance allowed (see 4.1.1).

b. Coil energizing voltage (see 4.1.1).

c. Cycling rate (see 4.1.1).

d. Contact bounce, if applicable (see 4.1.2).

e. If monitoring of contacts for sticking is not applicable (see 4.1.2c).

f. Whether final half of cycles is to be tested at room ambient temperature (see 4.2).

g. Rated maximum operating ambient temperature (see 4.2).

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MIL-STD-202-312

6. NOTES

(This section contains information of a general or explanatory nature that may be helpful, but is not mandatory.)

6.1 Supersession data. The main body and 38 parts of this revision of MIL-STD-202 replace superseded MIL-STD-

202.

Custodians: Preparing activity:

Army - CR DLA – CC

Navy - EC

Air Force - 85 (Project 59GP-2015-042)

DLA - CC

Review activities:

Army - AR, AT, AV, CR4, MI, SM, TE

Navy - AS, OS, SH

Air Force - 19, 99

NSA - NS

NOTE: The activities listed above were interested in this document as of the date of this document. Since

organizations and responsibilities can change, you should verify the currency of the information above using the

ASSIST Online database at https://assist.dla.mil/

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