This summary will explain the different types of relays available along with their benefits and detractions. Hopefully this will clarify some of the myths about relays.
Mechanical relays are standard in most kiln manufacturers’ products. Mechanical relays are the small black plastic boxes in the control box. The advantages of mechanical relays include relatively low cost and small size so they can easily be placed in small control boxes. This makes them the preferred choice for most kiln manufacturers.
They produce the clicking sound as two metal contacts come together and then separate. When the contacts are touching, the electrical current flows through the contacts to the elements, which generate the heat. Some kilns will click hundreds of times in a firing. The life of a mechanical relay is influenced by several factors.
First, the relay has only a certain number of clicks in its life. The more clicks in a firing, the fewer firings it will produce. Glass programs utilizing slow ramp rates and long holds cause the relay to cycle many times. Annealing ovens that hold for many hours also cause the relay to cycle frequently.
Second, the closer the relay is operated to its rated amps, the shorter its operational life. This stands to reason. Most mechanical relays are rated at 30 amps. If the relay is operated at 28 amps, its lifespan will be shorter than one operated at 8 amps. Most kiln manufacturers do a good job designing products, so almost all relays operate below 20 amps, and the majority are below 15 amps. This design preference cannot always be achieved, and some models may have mechanical relays used in circuit above 20 amps.
Third, relays with prolonged exposure to high heat environments will have a shorter lifespan and be more prone to periodic locking in the Closed or On position. Many shops have kilns located in areas without environmental control and poor airflow, which causes the ambient temperature to be well above 110 F. This is hard not just on the relays but also on the digital controllers. To mitigate this high ambient temperature, use a small fan to blow air across the electric components. It does not need to be a large fan. Small four inch to eight inch fans on low speed are very effective at lowering the temperature of the electronic components and thereby improving their lifespan.
Most kiln manufacturers have mitigated these potential drawbacks in their designs. Most kilns that operate at 30-45 amps, for example, have two to four mechanical relays. Each relay operates a certain set of elements. If one relay fails in the ON position, the remaining relays are OFF so the kiln does not receive more than 30-60% of the total power. This normally leaves the kiln to reach a steady state temperature far below fusing or melting temperatures for glass so a long anneal may occur. For example, one relay may fail ON and the kiln remains at 1000 F for an extra six hours. This normally does not adversely impact the glass, much less the kiln.
Mercury displacement relays (MDRs) are standard in few kilns, but some manufacturers offer them as upgrades. These are basically two vertical cylinders with a floating plunger inside. When the relay coil is activated, the generated electromagnetic field pushes the plunger up and completes the connection between the bottom and the top of the cylinder. The power is then allowed to flow through the cylinder to the elements. They address some of the detractions of the mechanical relays.
First, their expected life is in the millions of activations, which is far greater than that of mechanical relays. They have no physical contacts to wear out. Their failure mode is almost exclusively OPEN, not closed. This means almost no overfires. Since the relay works with the plunger moving upward in the cylinder when the relay is activated, gravity causes the cylinder to fall to the bottom of the cylinder when not activated, thereby opening the circuit.
The main detractions of MDRs are the use of mercury, cost, and large size. There is a small amount of mercury in these products, and we always dispose of these by returning them to the relay manufacturer. They recapture the mercury for later use and address all disposal issues. We encourage customers to return them to us for disposal. They cost about 4-5 times more than mechanical relays so they are not worthwhile in some applications, primarily small introductory models. However, the expense may be justified where the time and effort invested in the work outweighs the cost of a misfire from relay malfunction. Lastly, their large size precludes their installation in many kilns that are in the field, as many of the control boxes are too small for the MDRs. For many models, the metal control boxes are larger when MDRs are installed at the factory.
We are asked many times if the extra cost is worth it. Our normal analysis of the question follows these lines. Adding MDRs is not worth the expense to small intro kilns that operate on 120 volts and use a standard house outlet. The MDRs are very expensive relative to the price of the kilns. Moreover, the single mechanical relay in the small kiln operates at no more than 15 amps.
Medium size kilns that will be operated 5-7 days per week and are used in a studio environment for classes may benefit from MDRs. The studio owner may prefer to pay a little more rather than accept replacing mechanical relays and the possible delays in firing. Large studio kilns that fire projects with extensive preparation time are the main candidate for MDRs. The added cost of the MDRs is far outweighed by the potential loss of high value projects from relay failure. Many of the larger studio kilns come with MDRs standard and almost all have them available as upgrades.
Lastly, here are a few additional technical points on the use of MDRs. If retrofitting MDRs into an existing model, the MDRs must be mounted vertically so the plunger stays at the bottom of the cylinder when the coil is not activated. Also, most MDRs require the use of a mechanical relay to drive the coil, as most digital controllers do not have sufficient output power to operate the coil of an MDR. Therefore, the use of MDR does not eliminate the clicking from the mechanical relays, but the voltage through the mechanical relay is reduced to less than one amp so its life is very long.
Solid state relays are not used extensively in heating devices for two reasons: First, their predominant failure mode is the ON position, and they generate their own heat, which must be dissipated by use of a heat sync or fans on the control box. Most kilns used by hobbyists and studios have their control boxes mounted to the kilns. Kilns generate heat, which makes it more difficult for these relays to dissipate the heat in the control boxes. Excessive heat causes solid state relays to fail. Most ovens that use solid state relays have them mounted in control boxes that are remote to the kilns. These kilns are used in industrial application where the added costs of remote boxes is not much of an issue.
(This article first appeared in Glass Art magazine.)
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