Basics of Surge and Transient Protection – Part 5

In this multipart series we will cover surge and transient protection for all types of signal, control and power lines. My goal is to help the reader understand the basic principles and be able to assess the capabilities of the types of protection available on the market today. One of the most important skills you should learn from this series is the ability to ask the right questions and evaluate the answers received. In part 4 of this series we completed our look at clamp type devices. In this segment we will begin to look at crowbar type devices and how they are utilized in SPDs.

Crowbar devices are devices that change from an open circuit to a virtual short circuit when the voltage across the device exceeds a threshold voltage. Conduction of the device will continue until the current through the device falls below a holding current threshold while in the short circuit state. Since the voltage across the device, once it has triggered, is significantly lower than the normal circuit voltage, there needs to be a mechanism to end conduction of the device once the surge is passed. For AC service this can sometimes be accomplished at the zero cross over. This is not true however in circuits that have a DC bias voltage which can supply a greater current than the holding current required to maintain device conduction. The benefit of a crowbar device versus a clamp device is that energy dissipated in the device is minimized and thus a relatively small device can divert a much larger surge current than is possible with a small clamp type device. The largest disadvantage of some crowbar devices is the amount of time required to change the device from the non-conducting to the conducting state. While initiation times for clamp type devices are in the nanosecond (nS) range many crowbar devices are in the microsecond (µS) range.

Types of crowbar devices include spark (air) gaps, carbon blocks, gas tubes, and thyristors. Each of these types of device have specific advantages and disadvantages. Variations is shunt capacitance, time delay, turn off are among the differences that must be analyzed by the circuit designer to determine the best match between the circuit and the type of protector. We will begin by looking at spark gaps and carbon blocks.

A spark gap protector consists of two electrodes spaced to permit an arc when a specified potential is placed across the surfaces. The spark gap is not a sealed device and operates at atmospheric pressure and thus is exposed to environmental effects. The electrodes may experience oxidation and corrosion. These factors contribute to the air gap's high nominal breakdown voltage, wide breakdown voltage tolerance, and poor impulse response. Very large spark gaps are found in power distribution systems and on radio transmitting towers where the tower itself may have a large RF voltage present.

A carbon block protector consists of a pair of carbon electrodes separated by a 0.003-0.004 inch air gap. When a voltage is present across the terminals that exceeds the dielectric strength of the material in the gap an arc occurs between the terminals. For most carbon blocks this dielectric material is air. The voltage across the gap is limited by the arc and approaches zero volts. Follow on currents may continue long after the passage of the surge and may irreparably damage the carbon block in poorly designed circuits. Like the air gap protector, the carbon block is an unsealed device and its performance suffers in the same manner as the air gap. Another disadvantage of the carbon block is that it is made from a soft material that is degraded when limiting even a minor surge. This degradation typically causes the device to clamp at higher voltages during subsequent surge events. Carbon blocks are generally inexpensive and have been successfully used for many years to provide primary protection for telecommunication circuits.

The interface unit on the exterior of many residences and commercial facilities may contain one or more carbon blocks to provide a level of protection. In general, the telephone service provider has little interest in providing sophisticated protection for the telecommunications equipment in your home, office or industrial facility. They are generally interested in protecting equipment that they provide and in meeting their tariff requirements at the lowest possible cost. Damage to customer owned premises equipment is generally the responsibility of the customer to repair. Continuity of service may be drastically compromised if the TELCO customer does not take steps to protect their premises equipment. We will address this topic in greater depth in later segments when we begin to look at hybrid protection.

Next segment we will look at another crowbar device the gas discharge tube,