One of our readers asked us this question. “What needs to be grounded in a fiber optic network?” The standard answer of “everything” seemed illogical and was unsatisfactory to him.
Our research so far has yielded many more questions than answers. We want to share these questions and our impressions with our readership.
Good bonding and grounding has long been an indication of quality craftsmanship in the outside plant (OSP) network. In copper cables, bad things happen if we don’t do it.
• There are safety hazards.
• The cables become susceptible to power influence and other external noise issues.
• The cables can become hard to locate underground.
• The metallic sheath deteriorates.
Because of the capacity of fiber optics, many folks assumed that the bonding and grounding requirements should be higher than copper. “If we silver-plate our copper plant, we should gold-plate our fiber plant.” This is a great sentiment, but we rarely stopped to ask if we needed the same type electrical protection (or gold-plating).
It is now a common practice to install ground trees in sites that only include fiber optic connections. “Safety reasons” are the explanation, and, when pressed, National Electrical Safety Code (NESC) Rule 99 is cited.
Our research indicates that Rule 99 might not apply to these sites, and that this practice actually creates a potential lightning hazard where there would be none.
Question 1: If we had never worked with copper cable, how much bonding and grounding would we design into our fiber optic network?
We suspect that obsolete practices are being included in new practices out of habit.
Question 2: Is the NESC applicable to sites where there is no electricity flowing?
Because the NESC makes no differentiation between copper and fiber optic cables, it is commonly assumed that Rule 99 applies to fiber optic cable also.
That withstanding, it is logical to wonder why this national code would ever be intended to apply to a non-electric network. This code is not used to set water distribution standards. Why would it be used for light distribution standards?
Rule 99 begins with: When required to be grounded by other parts of this code. Where in the NESC does fiber optic cable meet any requirement to be included in Rule 99? We can’t find it.
Interestingly, Rule 97F applies, if the sheath is grounded. Simply put, we could be making Rule 99 apply by assuming it applies and grounding non-electric cables.
Question 3: Assuming that the NESC does not apply, is there a voltage hazard at non-electric sites that requires protection?
Our standards often surpass the NESC, and we are responsible for the safety of our people. Does grounding a fiber splice closure with no electrical connections really make the site safer?
We see a possible danger at these sites if we follow Rule 99 and ground the site. However, let’s assume that the sheath is bonded together at the connection points, but not grounded anywhere. Where is the electrical hazard danger? What path to ground would cause hazardous voltage to be drawn to glass cables?
Question 4: Why do we have metals in fiber optic cables, if we should not ground them?
We see 2 reasons:
1. For strength and damage protection.
2. A conductor for cable locator tone.
Grounding won’t make the cable any stronger, so it is difficult to see why grounding would help with that.
Until we can find a completely non-metallic means of locating buried cable under the ground, we must have a metal conductor to carry the cable locator signal. There must also be a ground on this conductor at the far end
for the cable locator current to flow.
Assuming that cable locating is the only reason to ground the sheath or wire, why not just do it during cable locates, and then remove the ground when the locate is completed?
Question 5: What about the optical to electrical interface points?
This one has a simple answer. With electricity flowing and a ground in place, Rule 99 of the NESC applies, and all of the electrical facilities at the site must comply.
With that point stated, does that mean that the fiber optic cable at the site should be grounded? Wouldn’t it be better to isolate the fiber optics from the electrical hazard potential completely? Furthermore, Questions 1-4 also apply to this question.
Some of our readers seem to enjoy the discourse when Vernon and I disagree. We find it more interesting when we examine a subject from completely different perspectives and come to the same conclusions.
Allow me to give you the long held McCarty policy on bonding and grounding fiber optic cable. First of all, we do not ground fiber optic cables. There is nothing to ground.
What we do is ground the fiber metallic shield, the metallic stress member, or the locate wire on one end. The only reason that we do that is to locate the path and depth of the fiber cable.
Our recommended procedure for locating fiber optic cable.
1. Go to the far end of the requested cable location area and ground the fiber metallic shield, the metallic stress member, or the locate wire to an independent ground such as an 8-foot ground rod that is not connected to anything else.
2. Go back to your end and connect the red lead of your cable locator up to the fiber metallic shield, the metallic stress member, or the locate wire; and the black lead of your cable locator to an independent ground
such as an 8-foot ground rod that is not connected to anything else.
DO NOT USE COMMON GROUNDS SUCH AS A POWER GROUND.
3. Measure the resistance of the far end ground to the near end ground. The resistance must be less than 1,000 ohms, and the closer to 0 ohms the better. If the resistance is higher than 1,000 ohms, then low frequency tone will not effectively run.
4. Set the locator transmitter and receiver to the lowest frequency setting that is below 1k. For example, 577Hz works just fine.
Note: If you use higher frequencies to locate such as 8kHz, 33kHz, 200kHz, etc., the locate tone will capacitively couple to other buried conductors, such as telephone and power cables, and you will get an inaccurate locate that endangers our cables.
5. WHEN YOU ARE DONE LOCATING, DISCONNECT EVERYTHING. FLOAT ANY METALLIC SHIELD, STRESS MEMBER, OR LOCATE WIRE. That will protect any metallic shield, the metallic stress member, or the locate wire from lightning and phase power.
There are some very smart fiber optic experts out there and we would like to hear from them. There are also brilliant attorneys focused on the interpretation of federal regulations, and they should certainly be consulted if there is any doubt regarding the requirements for compliance. Hopefully, one group or the other can answer these questions. The confusion on this matter is relatively widespread.
McCarty questioning the value of Bonding and Grounding might seem out of the norm. However, concerning fiber optic cable, we struggle to find justification for the labor and material expenses. This is especially true, if we are indeed making our sites more dangerous in the name of safety.
We teach safety practices during all of our classes. We know that unsafe practices in our industry can get you killed. We always advocate following all approved safety practices. In this case, we are simply questioning when and where the NESC applies to fiber optic cable in our environment specifically.
Please share your answers or other questions with us. The good answers received from our readership will be shared in a future column. The good questions will drive future subjects to investigate. We hope you are enjoying ISE EXPO 2016. If you are reading this while you are at the conference and want to discuss it, give us a call: Don McCarty 831.818.3930 or Vernon May 254.979.4749. You can also email us at email@example.com or firstname.lastname@example.org.