When the reed contacts open they break a circuit. If there is a voltage and current present when the contacts open they will stop the current flow. Once the contacts open the voltage that was driving the current will reappear across the contacts. This is normally not an issue unless the voltage is generally higher than 250 volts. At this level and higher when the contacts attempt to open they will begin to draw an arc. The arc is over 2000°C and will melt some of the metal on the contacts. The higher the current flow the stronger the arc and therefore more melting of the metal. Some of this melted metal will transfer to the other blade. If enough metal transfers, the contacts will stick. Breaking more than 10 ma can becomes a real problem with the higher voltages. Quenching the arc refers to the stoppage of the arc and/or metal transfer. If you are having to break high voltages care must be advised to minimize the amount of current flowing at the time of contact opening to minimize the metal transfer. Minimizing the current will prolong the life of the contacts.
There can be several reasons for early switching failures:
- Using the wrong switch.
- Not understanding the true nature of the customer’s load. This is usually the biggest culprit. If there are any transient voltages or currents present during the first 50 nanoseconds, they could rapidly cause the contacts to stick. These transients may be from stray capacitance, inductance in the line and/or common mode voltages. Equally important and not to be overlooked is what voltage and current is actually being broken when the contacts open. Any healthy voltage and/or currnet present will chew up the contacts rapidly leading to sticking contacts.
When one opens the contacts, any inductance in the line will cause the voltage to peak. This voltage can be as high as several hundred volts. This will cause arcing across the contacts and can rapidly lead to sticking contacts.
The actual switching takes place during the first 50 nanoseconds. Any stray capacitance may be charged up to the load voltage or potentially common mode voltages at the time of switch closure. In this case the contacts will switch that voltage with its associated current. This can degrade the contacts very quickly causing them to stick. Adding resistance in series with the reed switch will reduce the inrush current and prolong the life.
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In relays which have two switches in series: If one of the switches loses its vacuum, it will have a low breakdown voltage. Two switches in series is used to achieve the additive effect of two 10,000 volt breakdowns adding to give over 20kV. So what has probably gone wrong is one of the switches has lost its vacuum, perhaps due to a small crack or a bad seal. Try to remove some of the epoxy on the end were the reeds are soldered together and then test them individually to see which one may be bad.
If the high voltage is still testing good, it sounds to me like they may have switched too much power and/or carried too much current. Carefully break open the reed switch capsule and look at the contacts to see if there is any sign of pitting or burn marks right on the end of the contacts where they come together when the contacts close. If you see this, you will need to find out exactly what the customer is applying to the contacts and/or what he is carrying across the contacts. There are a few things that the customer can do:
- Add some series resistance in the direct line of the reed switch to reduce the max switching current.
- Check for any added cable or wire that adds stray capacitance.
- Make sure there are no common mode voltages present.
- Also, in his application, is it possible to switch the contacts dry (no voltage or current) for maximum life. This may not be possible.
- If the customer is switching some capacitance, can they add some inductance in series?
All too often customers find their relays failing early in their life times that is often caused by the existence of common mode voltages. Common mode voltages usually arise from line voltages in the area or nearby a given circuit. If there is stray capacitance in the line it can become charged to the peak of the line voltage. If the line voltage is 240 VRMS this translates to potential peaks as high as 400 volts. Switching this voltage even though the stray capacitance is only, say, in the order of 50 picofarads, it will cause metal transfer on the contacts. This will eventually cause early life failures. Better grounding can eliminate common mode voltages. Reducing the stray capacitance will help. Also, adding some resistance in series with the contacts will reduce the inrush. Keep in mind all the damage occurs in the first 50 nanosecond upon contact closure.