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Mean Time Between Failures (MTBF) of Crouzet Products
Calculating Mean Time Before Failure (MTBF) for a solid-state relay is, at best, a tricky proposition. Unlike a typical electromechanical relay, there are several variables that will directly impact the life expectancy of a solid-state relay in a given application. These variables primarily revolve around the electrical characteristics of the application, such as load current, the duty-cycle of the load, the ambient temperature inside the panel, surge currents, etc. However, mechanical issues such as the mounting method, available airflow, and thermal interface between the relay and the panel / heat sink also impact the relay's life expectancy.

Therefore, Crouzet does not publish a "fixed" MTBF specification. Instead, we provide an estimated MTBF specification range based on historical observation. That is, we calculate the MTBF by taking the total in-service hours over the previous two-year period and dividing that by the number of returns we have received from the field over the same period of time (solid-state relays that were misapplied by the customer are not considered in the calculation). "In-service hours" is simply calculated by taking the number of products shipped over the specified period and assuming that they were in operation for eight hours per day over a five day work week. Then, to ensure that we maintain an adequate margin of error, we assume that only 10% of all field failures are returned to us for analysis and adjust the results accordingly.

The final result yields a MTBF rating of between 2 million and 40 million hours, depending upon the product family. However, for the sake of simplicity we state that the MTBF rating for our solid-state relays is >2 million hours.


What does the "Z" and the "R" mean in the GN3 series relays?
"Z" stands for zero voltage turn-on (zero-cross or ZC) while "R" stands for random turn-on.


What is meant by zero-voltage turn-on?
The relay will only turn on when the line (output) voltage is near zero.


What is meant by instantaneous or random turn-on?
When the control voltage is given the relay output will turn-on regardless of what the line voltage is.


In what application would I use a zero voltage turn-on v. a random turn-on relay?
Zero-crossing relays are used with resistive loads while random turn-on relays are used with inductive loads (motors, transformers, coils etc.).

Zero crossing relays may also be used with inductive loads, but consideration must be given to the power factor of the load. If the load is too inductive, then the output of a ZC relay may half-wave. If this occurs, than a random-fire SSR should be used in the application.

A random-fire SSR may also be used in resistive applications. Some applications require that the load only be energized for a portion of the AC cycle. This can be accomplished with the random SSR’s (due to it’s relatively fast turn-on time), given that an appropriate controller is available. However, the initial surge current will be significantly higher due to the SSR switching power when the line voltage is closer to peak.


Why does the output of the relay not measure zero Ohms on my Ohmmeter when I apply control voltage to the SSR?
All solid state relays require a minimum amount of voltage across the output before the SCR’s will conduct. Most multi-meters are designed to simply provide a small amount of voltage in order to measure the ensuing current draw. Therefore, when measuring a solid-state device, the meter will typically report an open circuit (or very high impedance).


I am using an AC output SSR for use with a DC load, but the relay will not shut off. Why?
AC output SSR’s are zero current turn-off relays and thus must cycle through the zero current before it can turn off. You must use a DC output relay to control a DC load.


I have an SSR that turns on but will not turn off. Why?
First measure the control voltage. The input of a DC input SSR must go below one volt to guarantee that the relay will shut off (10Vrms for AC input SSR). If there is less than one volt across the input (10Vrms for an AC SSR) or if the input is completely disconnected, then the problem most likely resides in the output of the relay.

The two most likely reasons for an SSR failing to turn off are: the SCR’s have been damaged, or there is an over-temperature condition. When SCR damage occurs, the relay will typically half-wave. This is due to the fact that usually only one of the SCR dies will become damaged. This can result from excessive surge currents, transients on the AC mains, or other electrical phenomena. To determine if this has occurred, allow the relay to adequately cool down (approximately 15 minutes) and connect to a small load, such as a 100W bulb. If the bulb is illuminated without the control signal, then the relay’s output is shorted. Likewise, if the bulb is only partially illuminated when the control signal is applied, then the relay is half-waving.

The relay will also fail to turn off when the output SCR’s exceed their maximum rated temperature of 125°C. To determine whether a relay is "running" hot, measure the temperature of the base plate near the mounting screw. The temperature should not exceed 80°C to 90°C. If the relay exceeds 80°C, then the heat sink of the application must be reevaluated. Also, ensure that there is an adequate amount of thermal compound between the base plate and the heat sink. However, it is also important to note that too much heat sink compound will also impede the relays ability to adequately transfer heat to the heat sink.


How do I select an adequate heat sink for my application?
A. You must first determine the amount of power being dissipated by the relay in the application. You can calculate this multiplying the load current by the voltage drop (Vf) across the output of the SSR. The Vf of a SSR can be found in its specification sheet or can be measured individually. Once this is determined, then you must calculate the difference between the ambient temperature within the panel during operation and 80°C (recommended max base plate temperature). Then, divide the temperature difference by the power being dissipated to determine the maximum thermal impedance of a heat sink for the application.

For example, a relay is carrying 50 amps in an application with a Vf of 1.1Vpk, resulting in 55 Watts of power being dissipated. The ambient temperature inside the panel is 35°C, leaving a 45°C difference between ambient and the maximum recommended base plate temperature of 80°C. If you divide the temperature differential of 45°C by the 55 Watts of power being dissipated, the result will be 0.818, or a 0.82°C/W heat sink for the application. To be prudent, you should always deduct 0.1°C/W from the result to account for the thermal compound used in the assembly, and then round down to the nearest tenth. So, a 0.82°C/W heat sink less 0.1° is 0.72°C/W. Rounding down, you now know that you need at least a 0.7°C/W heat sink for your application.

Keep in mind that this is the most conservative way to calculate your heat sink requirements. If the duty cycle is not 100%, then you may need less of a heat sink. The best way to determine the "head room" in your application is to measure the temperature of the base plate of the relay within the actual application, keeping in mind the 80°C recommended maximum base plate temperature.


What is the minimum input current necessary to turn on a solid state relay?
This varies from family to family. Most of our solid-state relays have a regulated input, which means that the input will attempt to draw it’s "nominal" current rating across the entire voltage range of the relay. However, some SSR’s still have a straightforward resistive input, which means that the minimum input current for an "on" state is dependent upon it’s input impedance.

The DC input GN series SSR’s will draw between 10mA and 12 mA (maximum of 14ma, minimum of 7ma) across the full 3-32Vdc input range while the GNA5 will draw minimum of 9.5ma to a max of 14ma. The AC input GN series SSR’s will draw minimum of 5mA to max of 10mA across the entire 90-280Vac input range while the GNA5 will draw a minimum of 5ma to a maximum of 8.5ma. The GA3 three-phase SSR’s also have regulated inputs, but require up to 20mA for the DC input models, and 10mA for the AC input models.


What is the torque specification for Crouzet’s solid-state relays?
Most of our relays use the same size screws.....# 6 for input and #8 for output and mounting. The torque settings should be high enough to retain the wires on the terminals, but not so high to distort the clamps. Likewise, on the mounting screws, they need to be high enough to compress the thermal interface material, but not so high as to distort the base plate. So long as the materials for the screws, terminals, and base plates are similar, then similar torque settings can be specified. Here are our recommendations:

for # 6 input screws: 12 in-LB, +/- 2 in-LB

for # 8 output screws: 15 in-LB, +/- 2 in-LB

for # 8 mounting screw: 15 in-LB, +/- 1 in-LB

Note: there may be circumstances where these numbers may not be the best, but overall, they are close and a good place to start.


What is a snubber network and when would I use one?
A snubber network is a resistor and capacitor in series and is placed in parallel with the output of the ssr. Typically snubbers are required when the relay is placed in an environment that has electrical noise. Most manufactures incorporate a snubber network into their ssr’s. Crouzet uses an inductor along with transorbs to fill this role instead of a snubber network.


What is minimum Off-State DV/DT?
The rate of rise of applied voltage across the output terminals that the SSR (AC) can withstand without turning on in the absence of a turn-on control signal. Usually expressed as a minimum value at maximum rated voltage in terms of "volts per microsecond" (V/us)


What is I²T?
It is the nonrepetitive pulse-current capability of the SSR; used for fuse selection. Expressed as "amperes squared seconds" (A²s) with a stated pulsewidth, typically between 1 and 8.3 milliseconds.


What is thermal resistance?
Expressed as "degrees Celsius per watt" (°C/W), this value defines the temperature gradient between the output semiconductor junction (Tj) and the SSR case (Tc) for any given power dissipation. Basically, it’s the difference in temperature between two points on an SSR dissipating a specific amount of power. Thermal impedance is used to calculate the temperature of the SCR die in an application with a known heat sink impedance and Rjb (resistance; junction-to-base plate). The Rjb can be found in the relays specification sheet.

For example, a 25A relay with a thermal impedance of 0.25°C/W mounted to a 2.0°C/W heat sink is carrying 15 amps in a 30°C ambient. With a Vf of 1.0Vpk, the total power dissipation is 15 Watts (15 amps x 1.0 V). With a 2.0°C/W heat sink, the total temperature rise of the base plate above ambient will be 30°C (15W x 2.0°C/W), which puts the base plate at 60°C (30°C rise + 30°C ambient). The difference between the base plate and the SCR die is 30 Watts x 0.25°C/W (Rjb), or 7.5°C. When this is added to the calculated base plate temperature, we find that the SCR die is operating at 67.5°.


Why do some SSR’s have off-state leakage current?
Leakage current comes from the snubber network. The amount of leakage current (typically 10-15 mA) is dependent on the size capacitor used in the snubber network. Crouzet standard SSRs DO NOT include a resistor-capacitor network in most of its relays, which results in a typical leakage current less than 1mA.


What is a transorb?
It is a small device much like a diode and it is used to protect electronic devices from transients. Transorbs work by clamping any excessive voltage to a certain limit. They do this by conducting when excessive voltage is present between the two terminals. For a short time, the transorb will absorb a high amount of power. The transorb also reacts within a few billionths of a second making it superior to any traditional solution.


What is a MOV and how does it compare to a transorb?
A MOV is a metal oxide varistor, which is used to protect a SSR from transients, which can cause the SSR to turn-on without the control voltage applied. MOVs are typically placed in parallel with the output of the SSR. The customer usually installs the MOV. After a MOV reaches its life expectancy it typically fails shorted. By contrast, a transorb typically does not fail shorted and is installed at the factory when the SSR is built.


Why would I need EMC compliance?
More than likely, you probably don’t need EMC compliance. The European Unions EMC Directive for electrical equipment being placed on the European market only requires this. However, products that are EMC compliant are typically more reliable, as they are designed to operate in harsh electrical environments.


Why are SCR output SSR’s better than Triac output SSR’s?
SCR die are more capable of handling inrush currents and electrically-fast transients. However, given the right application, triac output SSR’s are quite reliable.


When would you use a SCR SSR, Triac SSR?
Triac output SSR’s are general-purpose relays typically used for resistive type loads only. The SCR output solid state relays are used to switch resistive or inductive loads, especially those with high inrush currents.
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