Test Sample

Accelerated life testing is the process of testing a product by subjecting it to conditions (stress, strain, temperatures, voltage, vibration rate, pressure etc.) in excess of its normal service parameters in an effort to uncover faults and potential modes of failure in a shortened amount of time, (Wayne, 1980).

In order to accelerate the relay testing, various parameters were explored; in the end, it was decided to test the device at full voltage and current, ambient room temperature and maximum switching cycle specified for the device. This still resulted in test times in excess of two weeks in some cases. The test conditions that were used, tried to invoke as much as possible the actual in-use conditions for the relay. The notion to use inductive loading, that would have sped up the degradation process was initially revoked, as most relays are implemented with some degree of arc suppression. By using a resistive load, a benchmark may be established from which a comparison of contact life from inductive loading and increased arcing may be examined.

The relays specification is documented in table 4-1 below. From the table, the life of the relay contacts is in excess of 105 _{cycles with a resistive load and the mechanical}

components of the order of 2 × 106_{, initial contact resistance is below 100 mΩ (this}

takes into account the oxide that may be present on the contacts due to prolonged RLead A V Rcontact RLead Test Current Sense Current pA VR RLead VM RLead

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storage or from manufacturing debris, hence the high initial value) and the maximum operating speed is 20 times/minute.

Characteristics Item Specifications Contact Arrangement 1 Form A

Contact resistance (Initial) Max. 100 mΩ (By voltage drop 6 V DC 1A)

Contact material AgSnO2 type

Rating Nominal switching capacity (resistive load)

16 A 277 V AC Max. switching power (resistive

load)

4,432 VA Max. switching voltage 277V AC Max. switching current 16 A

Nominal operating power 400 mW (Standard type) Min. switching capacity 100 mA, 5 V DC

Electrical characteristics

Insulation resistance (Initial) Min. 1,000 MΩ (at 500 V DC) Measurement at same location as “Breakdown voltage” section. Breakdown

voltage (Initial)

Between open contacts

1,000 Vrms for 1 min. (Detection current: 10 mA)

Between contact and coil

4,000 Vrms for 1 min.

Temperature rise (coil) Max. 55°C 131°F, Max. 45°C 113°F (200 mW type) (By resistive method, nominal coil voltage applied to the coil; contact carrying current: 16 A, at 20°C 68°F) Surge breakdown voltage

(Between contact and coil) (Initial)

10,000 V Operate time (at nominal voltage)

(at 20°C 68°F)

Max. 20 ms (excluding contact bounce time.)

Release time (at nominal voltage) (at 20°C 68°F)

Max. 20 ms, Max. 25 ms (200 mW type) (excluding contact bounce time) (With diode) Mechanical characteristics Shock resistance Functional 200 m/s2 Destructive 1,000 m/s2 Vibration resistance

Functional 10 to 55 Hz at double amplitude of 1.5 mm

Destructive 10 to 55 Hz at double amplitude of 1.5 mm

Expected life Mechanical (at 180 times/min.) Min. 2×106

Electrical (at 20 times/min.) Min. 105 _{(at resistive load)}

Conditions Conditions for operation, transport and storage

Ambient temperature: –40°C to +85°C – 40°F to +185°F; Humidity: 5 to 85% R.H. Max. operating speed 20 times/min. (at nominal switching

capacity)

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The measurements of contact resistance were generated from an in-house developed test bed at Cranfield University. The test bed in figure 4.2 has been designed to allow continual cyclic testing of a relay at various currents and voltages, with the ability to vary the switching time. In-situ measurements of currents, voltages, temperature, pick- up time, over-travel time, the rebound duration, closing time and contact resistance are available for algorithm development. The test rig consists of the following major components: 1000 W resistive load, 50 A solid state relay, 16 A 30 V relay, bread board, data acquisition system connected to a computer. The prognostic rig is designed so that no other component will deteriorate quicker than the relay during the test. Each component is discussed below.

Figure 4.2. Relay test rig

Solid state relay: In order to allow a continuous current through the load in the test circuit and switch this current during testing, a 50 A solid state relay was applied due to its reliability under continuous operation. This was coupled with a heat sink to allow the dissipation of heat from the device.

Resistive load: In order to load the device in the test circuit, a suitable load needed to be employed. A ceramic bodied 1000 W, 1 Ω load was used to enable the current from the power supply to be directly proportional to that being switched be the relay under test.

Test Circuit Relay: a standard relay was used for switching the test circuit for the measurement. This relay allows the 6 V, 1 A test current to be switched in whilst the load current is switched out.

NI ADC card 24 bit for high resolution measurement & digital outputs Load 1 Ohm, 1 KW Solid state relay

for switching in load and test circuits

Relay under test

Relay for switching in 6 V, 1 A

measurement circuit

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Bread board: The relay under test was mounted in rigid bread board as the connections are PCB type. As well as being PCB mountable, the relay also has TMP spade terminals. This allowed connection of the load circuit through the terminals and the voltage measurement to be taken as near to the contacts as possible on the bread board.

Figure 4.3. Picture of the test relay showing the TMP terminals.

Data Acquisition: National Instruments LabView® was used for the control of the test and data acquisition. The data collection was conducted with a NI 9219 Universal Analog Input, 24-Bit module and the outputs used for control of the test circuit, load circuit and relay under test was via a NI 9472 card which are connected to an NI cDAQ- 9174 4-slot USB chassis.

For safety reasons due to the long periods the test rig would be left unattended and the potential burn hazard from the 1 Ω load, the whole unit was mounted on a PVC board and housed within a steel cabinet.

Figure 4.4. Test circuit for relay.

Relay_under_test 1mH 1Ω Coil_Pulse 0V 5V 1s 1s G + - I1 1A G Measurement_circuit 1mH 1Ω On_measure 0V 5V 1s 1s G+ - Load_Resistor 1Ω I2 18A G Load_circuit 1mH 1Ω On_load 0V 5V 1s 1s G+ - Contact_Resistance DC 10MOhm 0.000 V + - TMP terminals

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