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1. Ambient temperatures may cause the relay to fail in the following circumstances
(1) The insulation material may soften or melt in high temperature; the insulation material may crack in low temperature. In these two cases, the dielectric strength of the material may deteriorate and the relay fails.
(2) Under the alternating effect of high and low temperatures, the structure may get loose, causing the moving parts to shift positions, which will result in out-of-control of the suction and release and loose contact or non-contact of the contacts
(3) In low temperature, moisture may condense or freeze in the relay, causing its insulation capability to deteriorate.
(4) In high temperature, the coil resistance tends to grow and the operating voltage will rise as well, resulting in non-pull-in or loose pull-in, which will eventually cause the relay to fail.
(5) In high temperature, when the contact switches power loads, the arc breaking capability declines, the contact corrodes, and metal transfer accelerates, resulting in a higher possibility of failure and a shortened service life.
(6) Temperature changes may cause the performance parameters of thermal relays, solid-state relays and hybrid relays to be unstable. The range of ambient temperatures of relays is usually determined by the product structure design, the performance of materials used and workmanship. The users' instruction manuals must be carefully read when purchasing the product. The temperature rise of the relay, especially the temperature rise of AC relays plus the maximum ambient temperature must be lower than temperature resistance level of the insulation of the enameled wire used. Full attention must be paid to this issue when selecting the proper relay.
Recommended temperature range is broken down into the grades :
Minimum temperature (°C) : -5±3; -10±3; -25±3; -40±3; -55±3; -65±3.
Maximum temperature (°C) : 40±2; 55±2; 70±2; 85±2; 100±2; 125±2; 155±2; 175±2; 200±2.
2. Moist Heat
Moist heat can be a threat to the performance of the relay in the following circumstances :
(1) Moist heat can directly cause the dielectric strength to decline and to fail eventually. If the relay is kept in exposed storage for an extended period of time or contaminated by sand and dust in the course of operation, its insulation may fail under the effect of moist heat.
(2) The coil wire of unsealed relays in moist heat may break off as a result of electrochemical corrosion or mould, which will in turn exacerbate the electrochemical corrosion and oxidation of the contacts and aggravate the corrosion of metal components, causing the performance and reliability of the relay to deteriorate, which will eventually result in the failure of the relay.
(3) The arc phenomenon may accelerate when the contact switches loads in moist heat, resulting in a shortened electrical service life. The issue of moist heat must be taken into consideration when designing or selecting materials for electronic products to be used in tropical and subtropical areas.
3. Low Air Pressure
Low air pressure may have the following adverse impacts on the relay :
(1) The insulation resistance and dielectric strength of insulation parts and components decline, the arc-breaking capability of the contact drops, and the service life is shortened.
(2) The radiation of the relay is damaged, and the temperature rise increases. This happens frequently in relays with big power consumption. However, low air pressure usually does not have a noticeable impact on relays for civil use and will not be discussed here in further details.
4. Sand & Dust
Sand and dust contamination may cause the relay to fail, which requires full attention of the users. Under natural ambient conditions or normal industrial workshop conditions, sand and dust may creep into the relays, especially those on the electronic devices in utomobiles, through radiating fins or cracks. Over time, the sand and dust may accumulate to such an extent that the rotating (sliding) parts will malfunction or be stuck, the electric contact of the contacts will fail, the corrosion of the metal components, and the insulation capability of the insulator will deteriorate or even fail in moist heat. Some electric protection relays and automobile relays may malfunction after one-two year's use despite the fact that they passed factory testing and inspection. The impact of sand and dust must be considered in the course of design and use. Users may specify their requirements to suit their actual needs.
5. Chemical Atmospheric Contamination
Organic vapor, oxygen, sulfur dioxide and salt mist in the ambient atmosphere have an corrosive effect on the contacts, metal components, coils and insulation parts of the relay, which will result in the poor contact or even failure of the contacts, the corrosion and breaking of the coil, and the deterioration of the insulation. Noxious chemical gas is omnipresent in nature; however, in different circumstances, the type of noxious gas (vapor) may be different. Technical precautions may be taken to reduce or eliminate the corrosion, but the cost is remarkable. Sealed relays for military use can have a leakage rate of 10-8pa.cm3/s if they are subjected to long-time high-temperature vacuum baking, filled with high-purity N2 and seal-welded with electron beam (or laser); the contacts can
be gold plated with 1-3u. To reduce the cost, relays for civil use are generally covered with a shell or plastic case to mitigate the corrosion of noxious gas (vapor) in the atmosphere. In real operation, depending on the load of the relay and the ambient conditions, the process vent can be opened to increase the radiation and to reduce the contamination of the contact surface by internal organic vapor and sulfur dioxide.
6. Mechanical Vibration
Relays in the vicinity of equipment with strong dynamic power or in transit may suffer vibration of acceleration value in a certain frequency range. Random vibration represents the field vibration stress effect created by missiles, jets with high thrust force and rocket engines.
(1) Vibration affects the relay in the following ways :
a. Vibration may cause the mechanical structure to be loose, fatigued, broken and fail.
b. Close contacts may fail because the vibration creates a transient break that exceeds the standard set time (10us, 100us); open contacts may fail because the vibration creates a transient close that exceeds the standard set time (10us, 100us)
c. Results in the relative motion of moving parts, causing noise, abrasion and other physical failure.
(2) Vibration Classifications :
Recommended vibration frequency ranges are as follows :
10-55Hz; 10-100Hz; 10-150Hz; 10-500Hz; 10-2000Hz.
10-5000Hz; 55-500Hz; 55-2000Hz; 55-5000Hz; 100-2000Hz.
Recommended amplitude (double-amplitude) and acceleration are as follows :
Recommended double amplitude (mm) for below critical frequency (57Hz) : 0.035; 0.075; 0.15; 0.35; 0.75; 1.0; 1.5; 2.0; 3.5. Recommended acceleration (m/s2) for above critical frequency : 4.9 (0.5g); 9.8 (1.0g); 19.6 (2.0g); 49.0 (5g); 98 (10g); 147 (15g); 196 (20g); 294 (30g); 490 (50g)
Relays may suffer mechanical impact in the course of transportation, handling and use.
The impact can affect the relay in the following ways.
(1) Impact may cause the structure to be loose, damaged or broken and lose its service ability.
(2) Due to impact, close contacts may fail for creating a transient break that exceeds the established requirement (10us or 100us) while break contacts may fail for creating a transient close that exceeds the established requirement (10us or 100us) Therefore, for Item (1), the relay must be capable of resisting impact, and the measurement results from mandatory items before and after the test must conform to product standards. For Item
(2), the relay must have an impact-resisting stability, and a dynamic monitor of the contact state of the contacts must be performed. Impact acceleration classifications (m/s2) : generally adopted are 147 (11ms), 294 (18ms), 490 (11ms), 490 (3ms), 980 (11ms), 980 (6ms), 1960 (6ms), 1960 (3ms) …
The ability of the relay to operate normally under the effect of constant acceleration stress must be tested. The effect of constant acceleration is not a concern in relays installed in regular ground electronic equipment and will not be discussed in details here. However, the effect of the constant acceleration in relays installed in aerospace electronic devices must not be overlooked.