TRANSIENT CONTROL LEVELS 

A new concept licks an old telco problem 

Overvoltages are here to stay, and they cause more problems than ever 

with sensitive electronic gear. The industry, can, and must cope. 

T ransient overvoltages are here 

to sta ; we can only learn to 

live wit{ them. The electrical 

and electronic equipment of yesterday could survive the effect of 

these transients because of con- servative designs with built-in 

capabilities. But today and tomorrow's equipment depends on sophisticated, miniaturized solid 

state devices. Because they are very sensitive to overvoltages, 

these devices cannot survive 

transient overvoltages if not protected. 

The transients we are discussing here occur on power systems as well as on signal lines. They 

are overvoltages caused by 

natural events such as lightning or high-voltage wires falling on lower voltage lines, and by such 

man-made events as load switching, intermittent contacts, or coupling from adjacent circuits. 

While transient duration is extremely short-a few millionths 

of a second (microseconds)- 

transient amplitude can exceed 

10 to 20 times that of the normal 

circuit voltage (see photograph). 

This overvoltage can cause component failure, sparkover between wires followed by a power 

arc, and mistrig ering of solid 

state devices. Ttis damage to e uipment, on the power supply 

s%e as well as on the signal lines 

side of the telephone plant, is 

the main issue here rather than 

the nuisance effects. 

Successful survival depends on a technically sound, costeffective coordination among the 

environment (transient occur- rence), the equipment desi n (withstand capability), and tfe use of protective sup ressors (voltage clamping). ~ucf a coor- dination has been developed in 

the electric utility industry with 

great success, but the electronics 

industry as a whole, including 

*F. D. Martzloff is in the Corporate Research 

and Development Department of General 

Elechic in Schenectady. N.Y. He is actiue on many ~tional committees inuolued in deagning protection deoiceo and setting standards. 

the communications people, has 

not yet been able to develop, to 

accept and to enforce a rational 

system of coordination. The 

Transient Control Level (TCL) is a plan for systematically coor- dinating the three parameters of 

environment, equipment and 

protection. 

The TCL approach proposes 

four specifications: 

1. Equipment will be designed to withstand well-defined 

"proof tests, rather than inconsistent simulations of an uncer- tain environment. 

2. Both the power lines and 

the signal lines of an electronic 

black box will be subjected to 

proof tests. 

3. A limited number of tests 

levels will be established; from 

the range of these the manufacturer and the user together will 

select a articular level for a given appYication. 

4. The waveshape (the value 

of the overvoltage as a function 

of. time) and energy content of 

the transient will be defined by 

established guidelines. 

These four specifications for 

TCL are rooted in several hard 

facts. Users and manufacturers 

must accept the impossibility of 

simulating all possible transient 

overvoltages encountered by 

equipment in service. Nevertheless, equipment can be designed 

to withstand a standard transient, 

and experience has shown that if 

the standard is set properly, the 

field experience of the approved 

equipment is far more accept- able than that of equipment 

which has not met the standard. 

Clearly, users would like to 

have high standards of withstand 

capability, while manufacturers, 

facing competitive pressures in an environment of undefined 

transient standards, would like to 

reduce costs by limiting withstand capability. 

Actuallv. ~roducine electronic 

equipmeLt kith hi -h inherent 

transient withstancf capability 

would be economic heresy. To 

allocate resources to a balance 

between moderate capability and moderate protection cost is, in fact, the rational approach to 

the problem of equipment proContinued on next page 

8.5 18 32 53 lo0 

Microseconds 

Osclllogram showing the warvoltages that can occur on a pow syslmn undu condlllons of 

swltehlng Wanslents. This was recorded wr s plod of 24 hwn at th. srvlm MbanM of a 

hwaa In whlch a mwcury swltch was swltchlng on and Man Ignition tmnstormr. 

SEPTEMBER 15,1977, TELEPHONE ENGINEER 8 MANAGEMENT 1 55 
tection. ~hf key concept of TCL 

is that the coordination of environment, equipment, and protection not only can be best accomplished but will be most 

cost-effective if it is planned 

ahead rather than retrofitted-if 

a retrofit is possible. 

In defining the standard test 

conditions, then, there must be a 

balance between the desire to 

produce high reliability and the 

desire to minimize cost. There is 

nothing new in this aspect of engineering. The novelty here is 

the fast pace of new equipment 

growth and of the progress in 

making transient suppressors 

available. 

Transient suppressors which 

can be applied in the telephone 

plant are of two types: the 

spark-gap devices and the 

energy-absorber devices. Spark 

gaps have been used for a long 

time in the forms of the familiar 

carbon-block and the more modem gas tube. In both devices the 

gas (or air) between two electrodes connected to the line and 

ground breaks down when an 

overvoltage occurs. The level of 

voltage together with the speed 

at which this breakdown occurs 

is a characteristic of the design. 

After the breakdown, a low voltage is maintained across the ap, which allows a high current 8ow 

without large power dissipation 

(hence heating) in the device. 

Spark gaps have the limitation 

that the breakdown requires 

some time to occur; until then 

the transient is not limited. 

While the delay may be no 

longer than a microsecond or 

less, there may still be time for a 

fast-rising transient to destroy a 

component before the spark gap 

responds (Sketch). 

Zener diodes, especially in 

their improved versions designed for transient suppression, 

and varistors (variable nonlinear 

resistors) are energy-absorber 

devices. In these the resistance 

of the device to the flow of current decreases as the current itself increases, a counteraction 

that produces a clamping effect 

on the voltage transient but that 

dissipates a large part of the 

transient energy within the device. Energy-absorbe r protectors 

do not have the delay limitation 

of spark Raps; they are permanently connected to the circuit 

and begin acting as the voltage 

rises, without waiting for a 

breakdown. Their limitation is 

the fact that the energy of the 

transient is dissipated in the 

source as well as in the device. 

The sharing of the energy is in 

inverse proportion to the impedance: for a iven device, a low 

source impe 8 ance forces more of 

the energy to be dissipated in 

the device, which has a capacity 

limited by its dimensions (and 

dimensions are generally limited 

by cost and by available space). 

Knowledge of the capability of 

suppressors to limit the voltage 

under defined conditions of 

overvoltage enables the equipment designer to build into his 

circuits the matching withstand 

capability. Survival is then assured. This is another key concept of TCL. Equipment is designed to fit the protective level 

that practical suppressors can 

guarantee not to be exceeded 

rather than to retrofit suppressors after the equipment is 

found to have transient problems. Of course this is an 

idealized situation because uncertainties of the environment, 

practical manufacturing tolerances, and the fundamental refusal of Nature to be coerced into 

neat human systems will still 

allow some failures to occur. But 

imperfect as the implementation 

of the TCL might be, it will still 

be better than to wait for the perfect system to come along. 

The TCL approach is being 

proposed to the industry through 

the medium of engineering society meetings, technical papers, 

and standards committees. Engineers are receptive to the concept, for the standard test conditions will enable them to compare results and to evaluate performance under consistt.nt conditions. While the concept has 

not yet been elevated to the 

status of an industry standard, 

acceptance is increasing. 

An example of possible coordination for cost-effectiveness 

can be found in the power 

supplied for tele hone electronics connectecf'to the AC 

power grid. Typically, rectifiers 

in these supplies are exposed to 

all the transients occurring in the 

mwer system: for central loca- 
the outset of a system. Telephone switchin circuits using 

hybrid relay anf semiconductor 

circuits are typically protected 

by gas tube primary protectors. A 

situation can develop where lack 

of coordination between the 

withstand capability of the com- ponents that immediately follow 

the protector and the clamping 

However, the failing component 

may in fact have acted, while 

failing, as a protector for the rest 

of the system, and raising its 

withstand level would let the 

transient go farther into the system, causing more extensive 

damages. What is needed is a systematic appraisal of the total 

capabilities and withstand 

the system. 

Protection for the more sensitive equipment being installed 

today is possible. Acceptance of 

the Transient Level Control concept throughout the electronics 

industry would enable telephone 

system manufacturers and op; 

erators to enjoy at reasonable 

cost a high degree of reliability.