ISE Columnist Don McCarty, OSP Expert

Delta Distributed Power and Its Effect on Telephone Circuits

Sept. 2, 2015
I received an email from one of my telephone friends, Ed Rousselot from the Megger Corporation, bonding and grounding experts, and in it he stated: From Ed Rousselot: Donald, You […]

I received an email from one of my telephone friends, Ed Rousselot from the Megger Corporation, bonding and grounding experts, and in it he stated:

From Ed Rousselot:


You may remember that during your session at OSP EXPO (2014), Chuck asked a question about why the telcos in California that are in areas served by Pacific Gas and Electric have been told to remove all attachments to what the telcos thought were (multi-grounded neutrals (MGNs). According to Chuck, PG&E says that the #6 wire coming down their power poles is not a true MGN, and having the telcos attach to them is a safety issue. You seemed familiar with that, and said something like "That’s because PG&E has (some kind) of power system." I can’t remember what kind of power system it was and why it was "special". Can you refresh my memory, please


My email response to Ed:


In many areas, PG&E uses a delta system. This system does not have a neutral return like a Y system, which has a neutral at least 4 times per mile. They have only 3 phase wires with no neutral. On a delta system the only common ground is at the network interface. Only at the network interface is the telephone ground attached to the power panel ground bar at the residence.

Ed, I suggest that you email Percy Pool who is an electrical protection engineer (PE), and he can explain it better than me, and he knows more about grounding than I will ever know. I have copied Percy Pool.

Keep me in the loop.


Percy responded to Ed and me. His comments are below, but first, a bit of information about Percy Pool. Percy is an electrical protection engineer (PE). He has spent over 25 years dealing with electrical protection issues including bonding and grounding of common infrastructures such as the distribution power system and the telephone infrastructure to protect the technicians and the public from being electrocuted because of a difference of potential between the 2 systems. Percy is well known in his field. He was a member of Subcommittee 2 (Grounding) of the NESC for almost 20 years, he was Chair and Vice Chair of ATIS’ Network Electrical Protection Subcommittee, and he is currently the Co-Chair of IEEE Wire-line subcommittee.

Percy’s response:

Ed and Donald,

The delta system (sometimes called unigrounded) is prevalent in California. The delta system consists of 3 wires (all hot) and there is no neutral.

Delta systems are grounded at the source transformer (generating plant, substation, etc.) and at the termination point which is typically the serving power transformer feeding the house. The National Electrical Safety Code (NESC) requires that the serving power transformer be grounded. Please note that in California they follow General Orders such as GO95 and GO128 (for aerial and buried facilities respectively) rather than the NESC. In a way the NESC and the GO documents are similar but the requirements in the California GO documents are sometimes stricter.

Delta systems usually have surge arresters (mostly for lightning protection but could be just for surges) along the power line. The number of surge arresters and their frequency (every pole, every other pole, etc.) is dictated by the utility company’s practices. The surge arresters are typically grounded to a grounding electrode (ground rod) via a 6 AWG conductor. Although this is a grounding conductor it is NOT a multigrounded neutral (MGN). Each one of these grounds may be considered "independent", that is they are not part of a grounding network such as the MGN. The NESC has very specific requirements for the MGN.

Telecommunication facilities (telephone or CATV cables) must not be bonded to the surge arrester grounds. If they were to be bonded then most of the surge that is diverted to ground by the arrester would go into the telecommunication facilities, causing serious damages. Some utility companies discourage the use of their delta pole-lines by telecommunication companies to avoid bonding incorrectly to the surge arrester ground electrodes. Some national standards discourage telecommunication companies from placing their facilities on power company pole-lines carrying over 5 kV (phase-to-phase) unless umbrella conductors are placed between the power conductors and the telecommunication cables.

Telecommunication facilities, whether on their own pole-lines or joint on power company pole-lines, are still required to be grounded, by the NESC, at least 4 times per mile. The CA GO documents may require more grounds per mile.

Telecommunication cables are required by the National Electrical Code (NEC) to be grounded at the Network Interface/protector at the house. This is usually accomplished by bonding to the power company ground rod or to a similarly approved ground point (such as the Intersystem Bonding Terminal).

When a telecommunications cable is routed within 6 feet of the power-serving transformer, the cable is to be bonded to the transformer ground. Also, when the telecommunications cable is placed within 6 feet of a power company ground (MGN), the cable is to be bonded to the MGN ground. Note that the NEC and NESC both require a minimum distance of 6 feet for the bonding to be required. Some other Standards and/or Company practices recommend bonding if there is 10 feet or less of separation.

Let me know if you need additional information.

Thank you, Percy. Percy’s email address is [email protected].

Signing off

Not knowing the above very important information has confused many individuals, including me, for countless years. If you have tips, concerns, or questions about safety as an outside plant copper cable tech, please let me know and we will see if we can discuss this in another column or in my newsletter. Be safe, be well, continue to do good work, and keep in touch. You can reach me at 831.818.3930 or email me at [email protected]. If you want information about my courses, email [email protected].

About the Author

Don McCarty

Don McCarty is the OSP EXPERT columnist for ISE magazine, discussing the issues around provisioning, testing, and maintaining copper for all services from POTs to IPTV. Don is also president of and the lead trainer for McCarty Products, a technical training and products company training field technicians, cable maintenance, installation repair, and Central Office technicians and managers. For more information, email [email protected] or visit