Allen-Bradley Bulletin 4983 Surge Protective Devices Application Note

Infrastructure for your Architecture Creating an electrical infrastructure that maximizes the reliability of your critical control circuits

Protect your Investment, Reduce your System Downtime Allen-Bradley Bul. 4983 Surge Protective Devices help to protect your electronic equipment to help extend the life of your investment. Surge Protective Device Background Information Transient Definition: A high voltage transient is a high energy, short term (1…10μs) deviation or change from desired voltage levels. It is an unwanted bundle of high electrical energy in the AC power line or communications line. The transient can be noted as a single overvoltage spike or a burst of spikes, sometimes followed by a ringing waveform. Causes of Transients: Any electrical power sources that switches on/off 20% Outside Facility • Lightning Strike • Utility Grid Switching 80% Within Facility • • • •

Large Starters / Motors / Contactors Variable Frequency Drives Air Circuit Breakers Welders

Effects of Noise and Transients (3 D’s): Process errors, System Downtime, Component Failure Disruption – Intermittent operation of the system/equipment. Examples include program and data errors, corruption of memory, and system crashes. Degradation – Components exposed to repeat transients and/or noise can experience shortened operating life. Over time, these disturbances degrade equipment, which can cause intermittent operation or corrupted data. The effects of transients and noise are cumulative. Destruction – Electronic equipment is sensitive to these transients and may fail. An example of this would be destruction due to damage on the printed circuit board traces.

Advantages of using Surge Protective Devices: • • • • •

Higher Surge Current Protection Rating Longer Equipment Life Expectancy Easy to Replace (when the device sacrifices its life) Less expensive to install and replace Remote and Visual Status Indication when protection has reached end of life

Surge Protective Device (SPD) Purpose: Primary purpose of any SPD is to provide protection from transients and surges. The SPD does this by clamping the transient voltage as quickly as possible, limiting the high-voltage spikes to an acceptable level, diverting the transient energy in a non-destructive manner to ground, all while allowing the expected energy to continues on its path. SPDs are wired in parallel and only activate when an event (surge/transient) occurs. SPD Technologies: In today’s industrial SPD marketplace, there are multiple technologies being used including silicone avalanche diodes (SAD), gas discharge tubes (GDT), and metal oxide varistors (MOV). Allen-Bradley products use a combination of all of these technologies, however the most widely used technology in the SPD market is the MOV.

MOVs An MOV is an electronic component with a significant non-linear current/voltage characteristic. When used in a surge protective device, the MOV is wired in parallel to the circuit and used to divert the excess transient energy to ground. Essentially when the circuit voltage is below a certain “activation” voltage the device’s resistance is very high therefore passing a low amount of current through it. However, when the circuit voltage is above the “activation” voltage the device’s resistance is low and therefore passes a large amount of current through it.

Specification Considerations when selecting Surge Protective Devices: When selecting or comparing multiple SPDs, it is important to understand the ratings that are being presented as in many cases these values can be complex and confusing. In this section, some of the key ratings will be described in order to help develop a better understanding of the significance of these values and what they mean. These ratings are shown in the 4983 Product Family Brochure in order to make the selection process easier for the customers as shown in the example on Table 3.

Table 3: Example of the 4983 Product Family Brochure Selection Table

Maximum Continuous Operation Voltage (MCOV): Maximum value of the voltage that can be applied before “activating” or “turning on” the SPD. Once this value is exceeded the MOV starts conducting thus shunting the excess energy to ground and “clamping” the energy to an acceptable level.

Voltage Protection Rating (VPR): UL 1449 maximum rated surge voltage value that will be allowed to pass through the circuit to the protected load device. This is the maximum value that is measured after the SPD has clamped/limited the surge voltage to the load. This is commonly referred to as the “let-thru” voltage.

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Impulse Surge Current Rating (Iimp): IEC rated peak onetime surge current value of a 10/350μs waveform that can be safely diverted through the SPD. This 10/350μs is an ANSI/IEEE C62.41 standard waveform that rises to 90% of the peak surge current value in 10μs and decays to 50% of the peak surge current value in 350μs (see Diagram 1). This is a very large energy waveform that can be considered a “simulated” lightning strike.

Diagram 1: ANSI/IEEE C62.41 10/350μs Waveform Diagram

Maximum Discharge Current Rating (Imax): IEC rated peak one-time surge current value of a 8/20μs waveform that can be safely diverted through the SPD. This 8/20μs waveform is an ANSI/IEEE C62.41 standard waveform that rises to 90% of the peak surge current value in 8μs and decays to 50% of the peak surge current value in 10μs (see Diagram 2). This value is used as a selection value for the desired protection level in most SPD offerings.

Diagram 2: ANSI/IEEE C62.41 8/20μs Waveform Diagram

Nominal Surge Current Rating (In): UL 1449 rated surge current value of an 8/20μs waveform (see Diagram 2) that can be safely diverted through the SPD at a minimum of 15 instances. UL declares the acceptable values for this test with a maximum value of 20kA.

Diagram 3: Visual representation of the energy level difference between the two test waveforms

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Publication 4983-AT001A-EN-P – June 2011

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