Breakdown Voltage and Lightning Surge for Relay Specifications
Breakdown voltage (also referred to as dielectric strength) and lightning surge breakdown voltage are two engineering terms used to rate the electrical robustness of a product.
Both terms are used in reference to insulation resistance; describing the maximum voltage that a product can withstand for a certain period of time before its properties are permanently changed. Hence it is a measurement of the product’s robustness to abnormal voltage conditions.
Although insulating materials have very strong chemical bonds preventing the flow of electrons, the strength of these bonds is finite. When an excessive amount of voltage is applied, the electrons will be slowly released and allowed to flow, degrading the integrity of the chemical bonds. As the product’s insulating strength is degraded over time, it begins to adopt the properties of a resistor, allowing current to flow.
Different methods are used for measuring breakdown voltage and lighting surge breakdown voltage.
For breakdown voltage measurements, an AC voltage source is applied and incremented at one minute intervals. As this voltage value increases, the insulation resistance of the product will begin to degrade, allowing current to flow. A detection current of 10mA is used as a maximum value to determine the initial breakdown voltage of a product. Therefore the values published in Panasonic’s data sheets refer to the initial breakdown voltage, at which 10mA or less is detected. It is understood that the breakdown voltage value will decrease proportionately with the change of the insulation resistance due to the material degradation and accumulation of contaminants.
Lightning surge breakdown voltage refers to the ability of a device to withstand a sudden externally generated voltage such as that from an indirect lightning strike, a power surge or other phenomena.
Lightning surge breakdown voltage measurements are taken between all mutually isolated conduction sections of a product. In a relay, these sections include measurements between:
1) Coil and contacts
2) Open contacts
3) Contact sets
4) Set coil and reset coil
In some Solid State Relays and all PhotoMOS relays, the breakdown voltage values are referred to as I/O isolation voltage and these measurements are taken from their corresponding input and output terminals.
Panasonic’s broad range of relays specify different values for the surge breakdown voltage. Most of these values are established by application specific industry standards that apply to communication network equipment, industrial machines and measuring instruments to name a few.
Therefore surge breakdown voltage values are normally specified with an impulse test waveform, indentifying three values:
1) Rise time – Time it takes the surge breakdown voltage to reach 90% of its peak value.
2) Peak value – The highest value of the surge breakdown voltage.
3) Fall time – Time it takes the surge breakdown voltage to decrease from 90% to 50% of its peak value.
In the graph below, the rise time is from 0 to 1.2 microseconds, and the fall time is from 1.2 to 50 microseconds.
In Panasonic’s datasheets, the rise time and the fall time are indicated in parenthesis as shown below for the TX-TH series:
Hence, the breakdown voltage specified between open contacts is 1,500 Volts, with a rise time of 10 microseconds and a fall time of 160 microseconds.
The surge breakdown voltage values specified by FCC part 68 and Telcordia are industry standards that Panasonic must comply with. These standards are implemented to provide a safety margin to equipment as well as personnel.