TRANSFORMER GROUNDING
Editor’s Note:
Years ago, I worked for a CEO who was one of the most gracious people I had ever known. It was said of him that there were only two reasons that people could get terminated from the company. One was abusive behavior to other employees, whether physical or verbal and the other was – improper grounding. Seriously! It was that big of a thing to him because it was all about safety.
“Work Safe, Home Safe” was the motto of my former company. One of the most important aspects of this motto was to make it something that was deeply rooted in our field crews. These were the employees most at risk from improper grounding of transformers they were going to be working on, and if our crews did not ground properly in their work, then other people and a lot more assets would be at risk. How did we make sure that the proper grounding procedures were followed? We made it a big deal when we communicated the importance that our CEO placed on the proper grounding of transformers; and we trained, retrained and practiced.
This short but very important article answers five critical questions about transformer grounding. We have added this to the Body of Knowledge on our website and I would suggest that if you have any employees or peers who should know the answers to these questions that you forward this article to them. It could save lives. Enjoy, and apply.
You hired someone to install and ground your transformer. You assumed they understood and followed all appropriate codes and considered the health and safety of your personnel and equipment. But what happens when they do not? Are you aware of some of the dangers that could be lurking due to an error or omission from the contractor? Our crews recently responded to an unfortunate example of just this scenario, involving transformer grounding.
We often focus on the importance of regularly testing and maintaining distribution transformers to preserve and extend their useful life. However, the life and safety of your electrical distribution system begins at installation. One of the critical elements of establishing a safe and effective distribution system consists of proper transformer grounding and connection of the system neutral in 4-wire systems. Those familiar with the National Electrical Code, NFPA 70, will recognize that there is significant time devoted to proper grounds within the code. Many do not understand why, nor the significance of these requirements.
1. What is the purpose of grounding the transformer?
Neutral transformer grounding serves as a permanent and continuous conductive path to “earth” with sufficient ampacity to carry any fault current and sufficiently low impedance to limit the voltage rise above ground, and facilitates the operation of the protective devices in the circuit. These functions reduce shock hazards to personnel and limit damage to equipment. Failing to properly connect and ground the system neutral in 4-wire systems establishes what is often called a “floating neutral”. In this condition, the neutral is not referenced to ground allowing voltages to “float” to a maximum of phase RMS voltage relative to ground. The amount of float depends largely on the load balancing of the connected system and can be particularly damaging to single-phase loads. More importantly, this condition creates an extremely dangerous touch potential hazard with the ability to injure or even kill.
2. When should I be concerned about proper grounding?
Recently, our service teams responded to a customer concerned about abnormal heating of a newly installed, refurbished transformer. Electrical testing replicated initial factory test results, but oil sampling showed combustible gases, which indicated overheating.
We often focus on the importance of regularly testing and maintaining distribution transformers to preserve and extend their useful life. However, the life and safety of your electrical distribution system begins at installation.