Doug's picture
High Frequency Measurements Web Page
Douglas C. Smith

 Address:  P. O. Box 1457, Los Gatos, CA 95031
 TEL:      800-323-3956/408-356-4186
 FAX:      408-358-3799
 Mobile:   408-858-4528

Technical Tidbit - January 2009
Injection of ESD Current Into Cables Using Capacitance
(a useful ESD troubleshooting tool)

ESD current injection using aluminum foil on cable

Figure 1.
Injecting ESD Current Capacitively Into a Cable Using Aluminum Foil

Abstract: When a piece of equipment is subjected to ESD, the cause of the resulting system response can be difficult to find because the ESD event couples to many parts of the system, not just the place where the event happened. A technique is described that allows ESD energy to be coupled onto system cables, one at a time. Doing so can identify sensitive cables that may be contributing to an ESD problem.

Discussion: Figure 1 shows a CAT5 Ethernet cable connected to a small circuit board (the other cables are not shown). The Ethernet cable is wrapped for several cm with aluminum foil and an ESD simulator is used to produce a contact discharge to the aluminum foil. This injects a significant amount of high frequeny current into the cable and as the voltage level of the simulator is turned up, one can determine the sensitivity to ESD of a particular cable. The amount of injected current can be determined by the following formula:

q = CV

by differentiating with respect to time we get:

dq/dt = i = Cdv/dt

For only one picofarad of capacitance and a 2000 Volt change in a nanosecond we get:

i = (10**-12 F) x (2000V/10**-9 sec) = 2 Amperes!

There is quite a bit more than one picofarad of capacitance between the aluminum foil and the Ethernet cable, so many Amperes of peak current will be injected. Some of the current will go to the left, down the cable, instead of into the board. We can minimize this current to the left by putting a ferrite core on the cable as shown in Figure 2. For a more detailed explanation of how current behaves with capacitive and inductive coupling, see my April 2005 Technical Tidbit, Inductive and Capacitive Coupling - Induced Current Characteristics.

ESD current injection using aluminum foil and ferrite on cable

Figure 2.
Injecting ESD Current Capacitively Into a Cable Using Aluminum Foil and a Ferrite Core

The basic method is to apply this stress to each system cable, one at a time starting at a low setting of the ESD simulator, about 500 Volts, and then gradually raising the level until a response is noted. If the response is the same as an ESD problem one is trying to fix, there is a good chance that the cable and the part of the system to which it is connected are associated with the ESD problem

With most ESD simulators, the foil must be discharged between contact discharges from the ESD simulator. This can be accomplished by connecting a few one megohm resistors in series to a suitable ground. Note that thin film resistors may be destroyed by ESD so use composition or other resistors that rely on the bulk properties of the resistive material and are physically large enough to prevent voltage breakdown.  The KeyTek Mini-Zap ESD simulator provides such a leakage path automatically.

Summary: A simple method was described for finding cables in a system that are sensitive to an ESD stimulus. The method is easy and quick to apply and can be used proactively to find ESD issues before formal testing, or to troubleshoot existing problems. This method should also be useful for problems associated with electrical fast transients such at those described in IEC 61000-4-4.

Additional articles on this website related to this topic are:
  1. February 2002, Cable Effects Part 2: Inductive Pickup by Cables in a System
  2. March 2002,  Cable Effects Part 3: Capacitive Pickup by Cables in a System
  3. April 2005, Inductive and Capacitive Coupling - Induced Current Characteristics
If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems.

Click here for a description of my latest seminar titled (now also available online as a WebEx seminar):

EMC Lab Techniques for Designers
(How to find EMC problems and have some confidence your system will pass EMC testing while it is still in your lab).

D-104 mike
Check out my podcast containing mp3 format short tutorials, tech news and more! Just click on the microphone to see the listing of free audio programs. Content is added every week  on technical topics so check back frequently.
Top of page

Questions or suggestions? Contact me at
Copyright © 2009 Douglas C. Smith