WIRE AND CABLE

Avoiding Magnetic Induction Issues in Communication Cabling

Surprisingly high magnitude currents can flow in metallic communication cables when these cables are in proximity to high current circuits. Such high currents could cause damage or even misoperation.

The following example shows how a primary current of 10,000 Amperes in one phase could induce 75 Amperes in a communication cable. This induced current could damage or affect relays, meters, and other equipment. This article offers guidelines to minimize the risk of problems in new and existing installations.

INDUCED VOLTAGE CAUSES LOOP CURRENT
Schweitzer Engineering Laboratories (SEL) has measured loop currents in communications cable located in metal-clad switchgear under normal load conditions. In one case, 10 Amperes of primary current unbalance resulted in 0.1 Amperes of circulating current in a communications cable shield. Therefore, during a 10,000-Ampere unbalanced fault, 100 Amperes would flow in the communication cable ground. This large current flow could destroy the communications cable shields, the communications port ground that the cable is connected to, etc. The large loop current might also cause communications failures or relay measurement errors during large fault currents.

Figure A. 1 illustrates the condition in which loop current is induced from a high-current conductor to a parallel, metallic communication cable. Assume there is current I flowing in a conductor, such as a power main cable in metal-clad switchgear. We also assume that the communication cable connecting the two devices is grounded at both ends. The current I flowing in the current carrying conductor produces magnetic flux (AmpereÕs Law). Electromotive force (emt) is produced when a metallic loop encloses magnetic flux (FaradayÕs Law). Since the loop is metallic, a current flows in it and the magnitude of this current is proportional to the emf and conductance of the loop.

Ampere's Law states that the magnetic flux produced by the current-carrying conductor is proportional to the current in the conductor and inversely proportional to the distance from the conductor.

Relating the magnetic field intensity, H, to the magnetic flux density, B (where µ₀ is the permeability of free space):

Substituting Equation 2 into Equation 1 yields:
Where:
B = flux density [Wb/m2]
µ₀ = 4 · p · l0^-7 [H/m]
I = fault current, I · sin(2pf) [A]
r = distance from conductor to point of interest [m]

Faraday's Law of Induction describes how an induced emf is equal to the rate-of-change of flux within an enclosed area.

Substituting Equation 3 into 4 and performing the derivative and integration operations yields:
Where:
f = power system frequency [Hz]
r = distance from conductor to center of loop radius [m]
s = loop area [m2]

To calculate the loop current divide the induced emf by the resistance of loop.

Substituting Equation 5 into 6 yields:
For a 60 Hz power system Equation 7 becomes

As an example, let us assume I = 10 A, f = 60 Hz (f), s = 1 m², r = 1 m, and R_LOOP = 0.01 ohm. For this example, the induced current in the metallic loop is 0.0754 A.

For a 10 kA fault current magnitude, and using the same loop area and distance from the current carrying conductor, iLOOP = 75 A.

Port Isolators Prevent Loop Current
Figure A.2 shows the transmit (TX) and receive (RX) driver pair of an EIA-232 communication connection between two devices. Note that both TX₁ and RX₂ are referenced to ground. Giving TX₁ and RX₂ the same reference through the shield (or ground wire) improves data integrity by minimizing loop area between the data signal and ground wires in the communication cable.

From the discussions above, we see that current can induce a small voltage into other circuits. As a quick solution for loop current flow in an existing installation, it might be tempting to simply cut the ground connection in one or both ends of an EIA-232 communication cable -- GND1 and GND2, respectively. This reduces the current flowing in the ground wire, because of higher loop impedance, but the communication channel becomes more susceptible to interference caused by increased loop area, degrading the communication channel ability to exchange data reliably.

We can prevent communication cable loop current with the use of an optical isolation barrier. With this approach, each TX and RX is referenced to the local ground. lt is important to know how your communication ports are configured and grounded. For example, the communications systems shown in Figures A.3 and A.4 illustrate installations requiring a port isolator at Device 1. In Figure A.4 notice that while TX1 and RX1 are isolated, chassis and remote ground signals are still connected at Device 1. The system shown in Figure A.5 does not require a port isolator, because the chassis ground of Device I is isolated from the ground of Device 2.

COMMUNICATION CABLE CHOICES
Fiber Optics
Fiber-optic cabling is immune to electromagnetic induction (EMI). With this type of cabling, there are no ground loops or ground potential rise (GPR) problems. Low-cost transceivers and fiber are available for use inside switchgear and substations.

Port Isolators
Port Isolator break cable ground loops and are useful in existing applications of metallic cables in switchgear. We do not recommend using port isolators for circuits outside the control house. Fiber should be used in such applications.

Metallic Cables
Metallic cables are adequate when all equipment is in one cabinet, or in control houses away from strong fields.

Avoid running metallic communication cables in the same tray, conduit, raceway, etc. with primary cabling. We also recommend exercising caution in routing communication cables near CT (current transformer) wiring where the CT can deliver high currents. If your communications must run in such places, then use fiber or at least a port isolator.

SUMMARY
Magnetic induction can induce large currents in communication cables when these cables are grounded at both ends, and the cables are near circuits carrying large currents.

It is therefore recommended that port isolators or fiber optics be used whenever magnetic induction is a possibility.

This article is based on Schweitzer Engineering Laboratories' Application Guide, Volume II, AG2001-06 by Jeff Roberts and Mark Weber.