Protecting & Grounding Communication Electronics
Transient Voltage Surge Suppression (TVSS)
The purpose of any transient protection scheme is survival of the protected equipment. Certainly quality TVSS equipment is a critical
factor. A quality grounding system is just as important. So far, most people have no trouble; however, questions arise when defining a
quality ground system.
There are two electrical characteristics of any ground conductor.
- DC resistance - This term is appropriate when dealing with fault current and other safety related issues. The National Electric
Code (NEC) stated requirement is a maximum of 25 ohms of ground resistance. Unfortunately, in the presence of a 3000 amp transient, we
can expect a voltage drop of 75,000 volts across such a ground system. Most equipment would not survive such voltages. Note: Military
standards for CE (communication electronics) facilities require 10 ohms or less (MIL-STD-188-124b).
- AC impedance - This term is similar to DC resistance with the addition of the reactive effects of the conductor. At transient
frequencies and with the kind of physical arrangements common to grounding systems, the inductive effect turns out to be the most significant
factor; therefore, we can expect the voltage drop to be higher than the 75,000 volts calculated due to DC resistance only. This is why we
talk about short, straight and large leads during TVSS installation.
Since the actual calculation of combined voltage drop due to inductance and resistance of a ground system is beyond the scope of
this analysis, we will jump directly to the voltage drop created by each component. This being the critical parameter for equipment
survival.
- Example 1:
- 25 foot #6 AWG ground wire (short ground wire from 60 Hz standpoint)
Inductance: L = 300 microhenries (approximately 1 microhenry per inch)
Resistance: R = 5 ohm ground (Very low ground resistance compared to NEC requirement)
dt = 8 x 10-6 second = 8 microseconds (rise time of standard test pulse)
di = 3,000 amp = peak surge current of standard test pulse
di
VL = Inductive voltage drop = L x ------
dt
3,000
VL = 300 x 10^-6 x --------
8x 10^-6
VR = I x R = 3,000 x 5 = 15,000 V
- Analysis:
- While the 15,000 volts due to a very low ground resistance is quite significant it is truly insignificant when compared to the voltage drop of 112,500 volts due to the inductance of a single ground lead.
- Conclusions:
- Attempting to reference all equipment within a facility to true earth ground is theoretically of no value and practically speaking, impossible.
Any grounding system that is going to be effective for transients must be very low resistance and have very low inductance. That is, short, straight and large.
A good alternative that meets these criteria is bonding between grounds of the various parts of a system. See discussion below:
- Discussion:
- Typical parts of a system might be defined as:
Power Supply -120 VAC input
Each telephone line or communication conductor
Each coaxial cable for services such as security cameras
Low voltage power supply conductors
Any of these or similar conductors must be protected as individual circuits, but from each other when they come together in some
common piece of electronic equipment. If this is not done then the piece of equipment becomes the surge suppressor, usually with
disastrous results. The alternative that field experience has shown to be successful is to electrically bond all of the grounds from
each of the different components. If this bond is very short then the voltage difference will be below the threshold of damage.
- Example 2:
- What is the ground resistance of the electrical system in a commercial aircraft at 40,000 feet? The obvious answer is infinity, there
is no earth ground connection at all. We all know there are many sophisticated electronic systems on board a modern aircraft, and they
work most of the time. What we tend to overlook is that all of these systems have a very low AC impedance ground bond via the aircraft
frame.
-
FIPS 94 says that a good high frequency (transient) ground bond must be less than 4 feet in length. If we assume the same 1 microhenry
per inch due to inductive effects, even this length will produce a voltage difference of 456 volts on our 3,000 amp surge. While many systems
will survive this level, some will not. Therefore, the axiom is the shorter the better!
- Conclusion:
- Even with excellent individual AC and Communication protection, if the respective grounds are not closely coupled, damaging
voltages can develop within the communication equipment. The customer assumes his costly suppression failed.
What does this mean to various practical applications:
- Small Building (less than 100 feet in any direction) with communication entrance co?located with the power entrance panel. All communication
circuits stay within the building.
Recommendation:
- Install panel protection using the normal system shield approach. Install central type communication protection at the common
service entrance.
- Result:
- Due to the common location, a short bond between the communication protector ground and the AC ground should be obtainable.
Since the building is defined as "small" there is a reasonable expectation that no damaging transients will be collected
internally.
Successful protection is expected.
- Any size building with communication entrance and AC entrance not co?located. All communication circuits stay within the building.
Recommendation:
- Install AC panel protection using the normal system shield approach.
Install combination AC and communication circuit point-of-use protection at all equipment equipped with communication circuits. See
example 1 below.
- Example 1:
- Using a plug?in protector to protect a FAX machine. The ITD protector device with RJ-11 establishes a Local Ground Window or ground
bond between the AC green wire ground and the ground of the telephone line suppressor. The voltage difference between the telephone
system ground and the AC ground is virtually zero, therefore, since both parts of the system are protected with quality TVSS, the
system survives. Since the FAX machine may be connected hundreds of feet from true earth ground, the AC ground impedance appears to be
an open circuit.
- Results:
- A short ground bond between communication circuits and the AC ground at each piece of equipment is assured.
Internally generated transients are also covered in this approach.
Successful protection is expected.
- Any size building, communication circuits leave the building (distance between buildings is not significant)
Recommendation:
- Install AC panel protection using the normal system shield concept.
Install combination AC and communication circuit point-of-use protection at all equipment which are equipped with circuits
leaving the building.
Treat other communication circuits as described above.
- Results:
- A short ground bond is assured.
Internally generated transients are also covered in this approach.
Successful protection is expected.
Additional information and site survey information can be obtained by contacting
POC: Mike Helms mike@lightningmike.com
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