When selecting a surge suppressor, one of the primary considerations must be the 'measured limiting voltage' of the device. This is defined as 'the actual amount of peak voltage that the load equipment is exposed to AFTER the surge suppression device operates', and is the critical factor in determining whether damage will occur to the protected equipment. In order to compare the performance of different transient voltage surge suppressors, it is necessary to establish uniform testing criteria, and to test under strictly controlled conditions. Whilst it is not possible to predict the exact nature of transient activity in any given installation, much research has been undertaken to establish 'typical' transient conditions experienced within the electrical environment. The ANSI/IEEE C62 standards define a number of such 'typical' events, together with a uniform set of test criteria in order to enable us to provide such comparisons.

Around 80% of the transient activity within our electrical environment is generated by switching on or off all internal loads. Whilst these events will generally be of lower magnitude than many external events, such as lightning or major power system events, they are still likely to cause problems with many of our sensitive electronic systems. Typical problems include premature equipment failure and data related problems such as PLC program difficulties and data communication errors. These lower level switching generated transients are typified by ANSI/IEEE C62.41 as the Category A1 100Khz ring wave, tested at 2000 volts and 67 amps, with the test conducted at 180° on the sine wave.

MEM offers two levels of protection: Threshold Suppression Network (TSN), or Active Tracking Network (ATN®). The Active Tracking Network (ATN®) is designed specifically to address the large number of switching generated transient events present in all our electrical distribution systems in order to protect the most sensitive electronic systems upon which we rely in our daily business activities. The selection of which option to use will be determined by the nature and the value of the equipment to be protected. The effect on the system of the different devices is illustrated below.

Unsuppressed System:

If we consider a single cycle of the 50Hz AC sine wave, at 90° the connected load equipment expects to see the peak voltage of the sine wave, at 180°, the zero-crossing point, it expects to see 0 volts, and at 270°, minus peak voltage. If we now apply the 2000 volt test waveform at 180° onto an unsuppressed system, instead of zero volts at this instant, the load equipment may be exposed to a peak overvoltage of 2000 volts. Obviously this has the potential to damage unprotected equipment.

Threshold Suppression Network (TSN):

This provides the best suppression of high energy impulse generated transients and suits a wide range of applications. As the name suggests, the suppressor has set thresholds above and below the peaks of the AC sine wave. Any deviation above this results in the device responding to clamp the overvoltage as close as possible to this threshold. With the same applied 2000 volt waveform at 180°, the measured limiting voltage in this test may be as high as 600 volts line to neutral. Whilst this offers excellent protection for the majority of our load equipment, and certainly all electro-mechanical devices, some particularly sensitive and critical load equipment may still be affected.

Active Tracking Network (ATN®):

In addition to the fixed threshold, this system now also has thresholds which track the AC sine wave, and are able to respond to any transient activity on the system at whatever polarity and phase angle it occurs. Typically this will reduce the same 2000 volt test waveform at 180° to around just 70 volts line to neutral. Clearly this now offers the best available level of protection for the load equipment, and is suitable for protecting the most sensitive electronic equipment such as PLC control systems, data communications equipment, laboratory instrumentation etc.