(Issue 17, 2010)
Why is my TVSS an SPD?
By Jerry Reaves,
In the last few years, the electrical industry has seen significant changes in safety standards. One area of major change is with surge protection. In the past, you would use a surge arrestor or transient voltage surge suppressor (TVSS) to solve transient overvoltage or surge issues. Today, we don’t even use the words surge arrestors or TVSS; we simply call them Surge Protection Devices (SPD).
The UL 1449 standard has progressed through several revisions and has become the standard governing all surge protection devices, including surge arrestors. One change to UL 1449 is intermediate fault current testing which has been added to test at 10A, 100A, 500A, and 1000A. In this test, the sample must either disconnect safely at the rated current or maintain the current level for seven hours without charring, flaming, or igniting a piece of cheesecloth lying on the test sample.
TVSSs and surge arrestors specifications are now combined into one single standard called Surge Protection Devices; this means that surge arrestors are required to pass the same testing as TVSSs. Previous versions of UL 1449 indentified two types of SPDs: permanently connected and cord connected. Now SPDs are identified as Type 1, 2, 3, 4 or 5. This change allows UL 1449 to align with IEEE standard C62.41.2 – 2002.
Nominal discharge current testing has been adopted from the IEC standards to test endurance and capability of the SPD. There are no grandfather clauses so all SPDs must be compliant and tested or retested to meet the 3rd edition as of September 2009. The 2008 National Electrical Code (NEC) Article 280/285 has also been updated to coincide with the latest UL 1449 3rd edition standard.
An explanation as to why all these changes have taken place begins with a review of the basic building blocks of an SPD. In the 1950s, selenium cells and gas discharge tubes were used. Selenium cells are very toxic, and in the 1980s were replaced by the Metal Oxide Varistor (MOV). Gas Discharge Tubes (GDT) have low capacitance, and are commonly used on high frequency lines. You can still find them in use from time to time.
Today the most common devices used are Metal Oxide Varistors (MOV) and Silicon Avalanche Diodes (SAD). The SAD is typically used in data/telecom applications and works very well, but it has limited fault current capabilities. The MOV is much more robust and is by far the most common technology in use today for transient overvoltage and surge protection. The MOV is made primarily of zinc oxide and is designed with a predetermined voltage threshold. The maximum voltage and current capacity of the MOV is determined by its physical diameter and thickness. When a nominal voltage is applied and is below the voltage threshold, the MOV has very high impedance and is not seen by the circuit. When the nominal voltage goes above the voltage threshold, the MOVs impedance will decrease and can go to almost zero ohms if needed, acting as a shunt to divert the excess voltage through the MOV. Typically, when an MOV is used in a surge protection application the MOV is connected across the main power lines and ground, which puts it in parallel to the load it is protecting. While the MOV is conducting the excess voltage it produces heat (P=I2R): the lower the resistance of the MOV, the higher the current level will be, and the more heat produced.
Ideally, when a transient voltage event occurs, the MOV clamps the voltage at the voltage threshold level and the load will only see that clamped level. When the spike has passed and the voltage level drops below the threshold level, the MOV returns to the high impedance state and stops conducting. This works well as long as the voltage level stays below the threshold long enough to allow the MOV to dissipate the heat and cool down. If the nominal voltage level rises and stays above the threshold for an extended period of time, or multiple voltage transients occur with little or no time between transients, the MOV may not have enough time to cool down. Depending on the severity of this condition, the MOV can overheat and burn up, catch on fire, or even explode. When an MOV burns, a mixture of toxic gases and soot is released into the environment around the MOV. The conductive soot can cause premature failures of exposed components..
Initially, you would expect the upstream circuit protection to open the circuit if this happens, but this is not always the case.
For example, let’s say the voltage is just slightly above the threshold point of the MOV. The MOV will start conducting but it will not produce enough current to open the circuit protection device. The MOV will continue to conduct, until it fails, which can cause a potentially harmful and unsafe condition.
Changes in testing procedures are challenging manufacturers to build better products that eliminate these conditions. The most common method of solving these problems is to add a thermal switching device to the MOV; so, if the MOV starts to overheat, the switch will open, removing the MOV from the circuit before it destroys itself or anything around it.
In a typical automation project, the sensitive devices that may benefit the most from SPDs include PLCs, operator interfaces, and communication devices. Other devices, such as transformers and power supplies, are not particularly vulnerable to transient voltage events. System designers should pay particular attention when designing systems that will operate in environments where inductive loads such as motors and contactors are turned on and off as part of normal operation or where variable frequency drives (VFDs) are being used, as this type of equipment is known to induce voltage spikes and other transients.
If you are already using Surge Protection Devices for your automation projects, make sure that you specify devices which meet or exceed the new standards. If you are retrofitting older equipment, make sure that you retire any selenium cells that may still be in use. Given the cost effective nature and multiple benefits offered by these newer SPDs, it would not be unreasonable to use SPDs in many applications to safeguard sensitive components and improve electrical safety.
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