Google Translate

Worldwide | Middle East

Electrical Discharge (ESD) Phenomenon Related to Telecommunications Cabling Systems

Introduction

Prior to discussing the electrostatic discharge (ESD) phenomenon and its effects on telecommunications cabling systems, let's briefly review the process of static electricity generation, charge storage and discharge.

Static electricity is usually generated by the triboelectric effect which consists of the contact of two bodies (materials), of which at least one is insulating, and their posterior separation.

Some materials tend to acquire electrons on contact whereas other materials tend to give away electrons more easily, then, producing charge exchange between the two bodies. After the separation, one body will tend to be negatively charged and the other one, positively charged. This process of charge exchange between two bodies is shown in Figure 1.

Charging can also occur by induction. In this case there is no contact between the two bodies. The inductive charging occurs when a charged body is placed in the neighborhood of a neutral or balanced conductor (from the point of view of charge distribution). Under this condition the electrostatic field will force the balanced charges present on the surface of the conductor to separate. The conductor will still remain neutral but with the same amount of positive and negative charges equally distributed along its surface.

Temporary ground of the neutral conductor causes some of its charge to leak away, thus leaving it charged without ever having been in contact with a charged body or material as depicted in Figure 2.

The typical process involved is the charging of neutral body and its posterior discharge (electrostatic discharge) through a conductor to another conductor which may be connected to the ground. The classical ESD phenomenon is that charging of human bodies due to the contact to low conductive materials (carpet as an example).

Equivalent circuits to analyze the mechanism of charge and discharge of a human body due to the triboecletric effect are depicted in the Figure 3.

The model in (a) is the simplest human body model whereas the model in (b) is the more complete human body model for ESD study. The body capacitance to ground is typically 150 pF and the body resistance can vary from 500 . to 10 k., depending on which part of the body the discharge will occur. Table 1 shows the relation between the part of the body involved in the discharge event and its associated resistance.

Depending also on the values of the body resistance and due to the triboelectric effect the body may be charged to a potential of the order of several kilovolts that will be accumulated in its capacitance. It has been found that human bodies can be charged to a potential of more than 10 kV [1]. When the human body is placed in contact with a grounded conducting body or material, the capacitor will discharge generating a current that will flow to the ground resulting radiated and conducted interference. Usually there is a small arc rising from the human body to the other body or material characterizing the ESD phenomenon. The model depicted in (b) includes the hand model as well as the body inductance, which is not always included in the model, but is very important for the determination of the discharge current decay time (capacitor discharge time).

Hence, the capacitance of the body is responsible for its charge storage. Capacitance, charge storage and electrical potential of a charged body can be related by the following expression

Where, C is the resultant capacitance of the system (in farads [F]); Qb is the electrical charge of the body (in coulombs [C]); Vb is the electrical potential of the body (in volts [V]) for the static field case.

When two bodies exchange electrical charges through direct contact or by induction, the total amount of charge present in the system, before as well as after the charge exchange, will be fixed. Thus, one can say that the charge unbalance of the system is fixed. Due to the same reason the product C. Vb is a constant. Placing the two bodies close to each other, the capacitance will increase and, consequently, the electrical potential will decrease. The opposite will also be true: separating the bodies the capacitance will decrease and the voltage will increase.

The typical ESD current waveshape is depicted in Figure 4.

The rise time is determined by the value of the body inductance and the decay time is a function of the values of capacitance, resistance as well as the dielectric constant and the conductivity of the body. The current Imax is limited by the voltage value of the body at the moment of the discharge. The rise time is too small as compared to the decay time and the typical relation is 1:100; for a rise time of 1ns, the decay time will be approximately 100 ns.

Telecommunications cables

Transmission lines or transmission channels can be described in terms of distributed network parameters such as resistance, inductance, conductance and capacitance per unit length. Generally, the wireline channel can be represented by the series resistance and inductance per unit length along with the shunt capacitance and conductance per unit length. A model (designated Model T) for an approximate equivalent circuit for a length dx is shown in Figure 5.

The parameters resistance, inductance, conductance and capacitance are considered to be uniformly distributed along the channel length and they vary as function of the conductors geometry as well as dielectric properties of the materials surrounding the conductors. These parameters can be calculated by means of the Maxwell equations or other suitable mathematical techniques.

Usually, the dielectric material surrounding the conductor used to separate the two conductors in the same cable pair has a very good frequency response so that the conductance can be considered negligible. The value of the conductance can be assumed zero for the frequency range defined for Category 5e cabling systems (from 1 MHz to 100 MHz) without compromising model accuracy. Considering this assumption and rearranging the wirechannel model, the resultant model can be drawn as shown in Figure 6.

It's important to realize that the wirechannel model is similar to the human body model for the ESD study, that is, both cases can be characterized by the same electrical configuration being composed by capacitance, resistance and inductance.

In order to represent a complete transmission channel (with a length l much longer than dx) using the T Model, a number of T sections (with dx length) have to be connected in cascade as depicted in Figure 7.

Similarly to the case of the human body, the cable capacitance will be responsible for the energy storage in the cable and its subsequent ESD. The cable capacitance can be computed by the suitable summation of several T sections connected in cascade to model a wirechannel of length l. The resultant capacitance of a cable pair will increase as the cable length increases and will decrease as the cable length decreases (see the equivalent capacitance for different cable lengths in Figure 8).

Capacitance of telecommunications cables is also referred to as mutual capacitance. Cable manufacturers use this term to specify mutual capacitance values for their products and these values are frequently published in their products' catalogs. In order to minimize the crosstalk effect between pairs, cable manufacturers use different techniques to control the value of mutual capacitance of their cables.

For higher cable categories, lower mutual capacitance values are observed. In other words, Category 5e and Category 6 cables have lower capacitance than Category 5 and Category 3 cables. Table 2 shows the mutual capacitance values for several telecommunications cable types and categories for different cable manufacturers.


ESD Events on Telecommunications Cables

Charge storage in telecommunications cables can take place by the following mechanisms (isolated or combined):

  • Conductive coupling;
  • Electromagnetic coupling.

Charge storage due to conductive coupling can occur when the electrical potential of the equipment connected to the cable in one of their ends is higher than the electrical potential of the equipment to be connected to the opposite cable end (see Figure 9).

Figure 9a shows the equivalent cable model for the cable length, l. ZL1 and ZL2 are the equivalent load impedances of active equipment of system 1 and system 2, respectively. V1 and V2 are the electrical potentials (voltages) of the load impedance of the systems 1 and 2, respectively (let's assume that V1>V2).

Considering the active equipment (system 1) connected to the one cable's end (Figure 9b), the voltage at the point A will be the same as the voltage V1. The equivalent cable capacitor, Cl, will then be charged due to this potential and the voltage at the point B will increase. Supposing that V1 increases due to any circuit's failure in the system 1, the voltage at the point B will also increase.

When the cable is connected to the system B, in order to interconnect systems A and B, the accumulated charge in the equivalent cable capacitance, Cl (responsible for the voltage at the point B) will create an ESD current flowing through the equivalent load impedance of the active equipment of system 2, ZL2 (Figure 9c). Depending on the voltage level at point B at the moment of the electrostatic discharge as well as the protection methods implemented in system 2's circuitry, the intensity of the ESD current will be high enough to cause some damage to it.

The electrical potential at the point B can reach values of several kilovolts depending on the channel length and the cable's mutual capacitance, largely based on the cable category. In the case of electromagnetic coupling, the discharging process will be the same as in the case of conductive coupling. The only difference is that the charging process will occur due to the electromagnetic field present in the environment where the cable is inserted.

ESD protection and control

Due to many similarities with electromagnetic compatibility (EMC), ESD protection and control methods are similar to those used to control electromagnetic interference (EMI) and are listed below:

  • Common mode chokes;
  • Overvoltage clamping devices (as capacitors, varistors, avalanche diodes, etc.);
  • Conductivity control of working surfaces;
  • Humidity control of rooms;
  • Cable shielding;
  • Cable bypass filters.

From the list shown above, the only method associated directly to cabling systems is the use of cable shielding. This method is very efficient but not necessary when used with active equipment with good interface input protection as well as suitable circuit design and PCB (Printed Circuit Board) layouts as part of the telecommunications network in a given installation. Table 3 below shows the voltage measured between the cable conductor and ground in one channel end when a discharge of 10,000 V occurs at the far end. In all situations presented in this table the shield has a 360° bond to the equipment box connected to the functional installation grounding. The shielding at the opposite end varies.

The active equipment protection can be easily implemented using the common mode choke technique and overvoltage clamping devices. The time response of the system protection is its key factor as a typical ESD event time duration is too short (typically more or less 100 ns).

Even so, if a spontaneous accumulation of charge occurs, this situation can be prevented by terminating unused network ports with shorting plugs or providing connecting hardware with shorting contacts that are physically moved out of contact by mating up the connection.

Conclusion

There has been no change in the cable insulation when going from Category 5 to 5e or 6. The materials are still the same that have been used in this industry for several years. Thus, the incidence of ESD in high performance cabling and associated active equipment is less now than in the past.

ESD currents generated by Category 5e and Category 6 cables in an electrostatic discharge event will be less harmful to the disturbed circuit than those generated by lower cable Categories in electrostatic discharge events.

The electrical potential (voltage) measured at the input of the disturbed circuit due to the static charging will be also lower for higher cable Categories.

The other factor of concern on the side of network equipment is that the susceptibility of this equipment to ESD is increasing due to the implementation of new transceivers employing more PHY chip density. The implementation of new electronics without suitable changes on the front-end regarding to the ESD susceptibility will also lead the electronics to damage due to the electrostatic discharge phenomenon.

Bibliography

Noise Reduction Techniques in Electronic Systems, Second Edition
Ott, Henry W. - John Wiley & Sons, Inc. 1988

Principles and Techniques of Electromagnetic Compatibility
Christopoulos, Christos - CRC Press Inc. 1995

Digital Transmission Lines, Computer Modelling and Analysis
Granzow, Kenneth D. - Oxford University Press Inc. 1998

Cable Shielding for Electromagnetic Compatibility
Tsaliovich, Anatoly - Van Nostrande Reinhold 1995


Shielded Learning Centre
Case Studies - See how Siemon is connecting the world to a higher standard
Find Partners

» Find Siemon Authorized Distributors

» Find Certified Installers & Consultants

Ask Siemon
Have you questions about cabling?
» Ask Siemon
Cisco and Siemon

Cisco Technology Developer Partner


See Siemon in Cisco Marketplace:

Category 7 Cabling?
Cat 7 for the real world Articles and case studies 48 pages
» Learn more