By Rick Hampton, Cutler-Hammer and Westinghouse Products; Russell Barss, P.Eng, Tycor International Corp.
With new technology requiring greater protection from voltage transients and electrical noise, there is an increased need for quality surge protection devices (SPDs).
Engineers regularly specify surge SPDs at panelboards and switchboards to eliminate surges and electrical line noise from a facility's electrical distribution system. SPDs (also known as transient voltage surge suppressors) protect solid state devices (computers, drives, PLC systems) from the damaging effects of voltage transients and electrical noise.
Suppressors are a standard feature on most specifications for industrial and commercial facilities. The growth of microprocessor-based equipment has increased the need for quality suppression devices. While there are a range of competing designs, knowledgeable consultants choose suppressors that are UL1449 listed, have adequate surge current capabilities and minimize let-through voltage. In addition to these basic attributes, specifications now contain four key criteria. The suppressor is installed (integrated) into distribution equipment. The internal circuitry has both suppression and filtering elements (hybrid design). Internal fuse protection is standard on all devices, and advanced monitoring capabilities provide more reliable status indication.
Integrated Suppression Provides Enhanced Performance
Historically suppression devices were purchased as stand alone devices and mounted next to a
panelboard or switchboard, with cable connection to an available circuit breaker (see figure 1). These
designs are still widely used and ideal for retrofit applications. In the past few years, however,
switchgear manufacturers began integrating suppression devices inside electrical distribution equipment
such as switchboards, panelboards, busway and motor control centers. Due to the integrated design,
many of the problems associated with stand alone devices are eliminated. The most significant benefits
are improved suppression performance and the elimination of field installation problems.
Directly connecting the suppressor to the bus bar reduces impedance in the surge path, resulting in significantly lower let-through voltage (let-through voltage is the surge remnant that passes through a suppression device and attacks sensitive microprocessor-based equipment).
In many cases, the installation lead length for an externally mounted unit is over 14 inches away from the bus bar. The impedance associated with this lead length increases let-through voltage when measured from the bus bar.
Depending on the wiring configuration and inductance effects, lead length will increase let-through voltage between 15 and 30 volts per inch. By directly connecting the suppressor to the bus bar, and eliminating the installation lead length, let-through voltage can be reduced by approximately 50% (see figure 2).
The integrated design also eliminates field installation costs and expensive "outboard" wall space. It will minimize potential electrical code violations associated with improper installation procedures. The need for post installation field testing of the suppressor will be reduced as well as the invalidation of third party certifications when connecting to existing products.
Filtering Eliminates Low Level Disturbances
A "basic" suppressor is constructed using only metal oxide varistors (MOVs). These nonlinear components shunt surge current when the voltage exceeds approximately 200 per cent of the nominal voltage.
More advanced designs are hybrid suppressors which contain both MOVs and filter components mounted in parallel. Filtering (often called sine wave tracking) reacts to low level transients at any phase angle and "turn on" much earlier than the MOVs.
The most common type of surge found within a facility are ringing transients. A high performance filter is effective at removing these surges (see figure 3). This level of performance ensures microprocessors are not upset by continual bombardment of low level surges.
In addition, the filter should attenuate electrical line noise. To ensure effective filtering, specify at least 40db attenuation at 100 khz (using MIL-STD-220A insertion loss test). Note the 100 khz frequency represents the most critical region for removing noise from a.c. distribution systems.
Internal Fusing Capabilities
Properly co-ordinated internal fuses are a necessity to maintain a safe operation. Fusing systems must
be capable of shunting high energy surges as well as immediately opening should any internal components be damaged. This will protect the integrity of the electrical distribution system from a suppressor
that has failed and created a short circuit on the system. Not all suppressors come equipped with internal fusing capabilities.
Comprehensive Monitoring Systems
Customer requirements for suppression monitoring have expanded in the last few years. Suppressors
that do not monitor the status of the device are no longer acceptable. More advanced internal monitoring and diagnostic systems have been developed that provide maintenance personnel with reliable status
indication and operating data. For example, customers are now requesting systems that provide continuous monitoring of all modes, including neutral-ground failures. Monitors must have back-up fault detection, maintenance free operation and real-time response to trigger remote alarms.
These new monitoring systems ensure the investment in transient protection is working as intended by the manufacturer.
Suppresion designs are now available to ensure that the investment in power protection is not wasted as a result of poor suppressor design or installation. A quality suppressor is integrated into the distribution equipment using a minimal lead length, low impedance connection. The unit incorporates suppression elements and hybrid filtering to properly protect against the damaging effects of transient voltages and electrical noise. Internal fuse protection and a continuous on-line monitoring diagnostic system ensures safe and reliable operation. Designers and end users can now create electrical distribution systems that deliver the quality of power required to maximize the performance in today's microprocessor world.