Technical Notes
Switching Loads and Electrical Service Life
Relay switching performances are affected by high ambient temperatures, humidity, dust and contaminent gases. A relay itself creates heat and oxidants as it operates. These pages are intended as a guide for the general user operating between -10°C and +40°C. For more data on this subject please contact us.
The main influencing factor apart from the above, on relays electrical service life is the arc produced when the contacts open and close. Contact friction, clearance mechanical quality etc are of lesser significance.
Closing Arc
As a contact is activated, and two points come together to carry current, an arc will form. This causes material to evaporate, and if high transient currents are present (say starting a motor or a fluorescent lamp) then large portions of the contact surface may melt causing the contacts to weld. The process is reinforced by contact bounce. As the contacts close the arc is suppressed; it appears as a peak.
Opening Arc
To break a circuit, a contact will open. As it does so the effective contact surface is reduced due to the decreasing contact force and movement. The current flow is the same, and therfore the current density in the remaining pathway rises up to melting point. An explosion like process can occur as the contact material springs out. An arc may be produced by:
- resistive and capactive loads in conjunction with high voltages
- inductive loads
Permanent arcs are producd mainly by DC current. Alternating current quenches the arc when the current crosses the zero point. Arcs are influenced by contact material, a reduction in arcing voltage and arcing current and the speed of the the switching elements. When switching high DC loads, a larger contact gap and blow out magnet may be critical(these can be found in the Kuhnke IA relay range).
Inductive Load such as Motors
An electric motor will draw a very high current in the start process. This leads to a high make current as relay contacts close together. The load will vary according to motor size and design, but peaks of 15 times FLC (full load current) are not unusual. Industrial relays will be operating at their design limits close to the normal FLC and will often not be suitable for the start peak. A relay, with some exceptions is different to a "contactor" or motor starter. If in any doubt please contact us.
Lamp Loads
The resistance of cold filament bulbs is only about 5 to 10% of the value measured at operating temperature. The making (or "inrush") current is therefore 10 to 20 times higher. A 100W bulb for example has an inrush power of more then 1000W. Look for AgSnO contacts (or better) on your relay, and check the nominal current capacity very carefully.
Protective Circuits
To reduce load on contacts at breaking voltage peaks, there are several options.
Protection Diodes have a breaking voltage peak of 0.7V, and
no effect on making behaviour. They do delay drop out by 3 or 4 times.
Varistor Circuits are suitable for AC and DC and include a
polarity safeguard. They protect from high breaking voltages with minimal
addtitional drop out delay. They have limited switching frequency and are optimised for
only one voltage.
RC Elements apply to AC and DC, with minimal delay and
polarity safeguard. A low overvoltage setting can be achieved. Not normally
effective with small voltages. A disadvantage is the capacitor (C) can lead to
high making currents (NB RC = Resistor + Capacitor).
Suppressor Diodes apply to AC and DC circuits with minimal
drop out delay and a polarity safeguard. They have limited switching frequencies and work
with only one specified voltage.
To speak directly to experienced relay professionals please contact.
The Relay Company
H. Kuhnke Ltd
21 Abbey Enterprise Centre
Premier Way
Romsey, Hants, SO51 9AQ
UK
Telephone: +44 (0)1794 514445
Fax: +44 (0)1794 513514