Understanding Transformer Grounding: Solid Ground

As promised in the last article, this “Solid Ground” is about transformers and grounding. First, let’s start with some definitions. Opinions differ on the “official” definition of a “distribution” transformer. However, the United States Federal Register (Vol. 71, No. 81; page 24,995, published April 27, 2006) defines a distribution transformer as one that meets all of the following criteria:

• It has an input voltage of 34.5 kV or less
• It has an output voltage of 600V or less
• It is certified to operate at a frequency of 60Hz
• It has a capacity between 10 kVA and 2500 kVA for liquid immersed transformers or between 15 kVA and 2500 kVA for dry transformers

This definition specifically excludes autotransformers as well as operating (isolation), grounding, machine tool (control), and non-ventilated (dry) transformers, among many others.

While vague, this definition suggests that a distribution transformer is generally utility-owned equipment found in substations or on the utility side of the grid. Most of us remember having them once called power transformers. The current widely accepted definition appears to be that a distribution transformer serves an electricity supplier customer while a power transformer serves an area.

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According to article 100 of the National Electrical Code (NEC), the electrical panel is the only point through which electrical energy enters a facility. Shunt equipment typically includes circuit breakers, circuit breakers, fuses, and related accessories, and connects to the load side of the spur conductors. This connection equipment forms the main power and control switch, but does not include a meter.

According to the NEC, spur conductors originate at the power service point and terminate at the line side of the spur equipment. Power conductors are considered conductors and equipment on the load side of branch equipment, such as secondary conductors of customer-owned transformers, generator, UPS or photovoltaic (PV) system conductors, and conductors serving distant facilities.

A separately derived system is “a field wiring system whose power is derived from a source of electrical energy other than a service input. These systems do not have a direct electrical connection, nor a solidly bonded grounded loop conductor , to supply conductors from another system,” according to the NEC. Separately derived systems include, for example, transformers where the (primary) supply is isolated from the secondary supply, except when magnetically coupled, generators (self- or alternative source) where the (neutral) conductor put ground is not securely connected in transfer switch, battery/inverter systems where the output is not interconnected, and stand-alone PV systems.

Grounding and connection

Grounding means to connect something to the ground. To bind means to connect objects together. A separately derived system must be grounded at source. The equipment and metal parts not subjected to current must be connected to the earth point of the branch system. Bonding metal equipment creates an efficient current path in the event of an earth fault and protects the electrical system from the risk of electric shock and fire. Proper grounding of branch systems separately stabilizes the voltage between phase and ground. NEC Article 250 in general, and Article 250-26 in particular, address the requirements for grounding a branch system separately.

Proper grounding of a transformer is critical. Placing a ground connection (usually to structural steel, which should be connected to all cold water pipes) establishes a reference ground. Make a proper connection by exothermic welding, without using pliers, which can loosen over time. Make sure that the high-frequency impedance of the ground conductor is as low as possible. Large, flat conductors show less inductive reactance at higher frequencies, which is why they are preferred over round conductors. The distance between the neutral-earth (NT) connection of the transformer and earth must be as short as possible.

Neutral and ground must be connected to the transformer neutral bus. To isolate normal back currents from earth currents, it is not recommended to configure the NT link on the main panel. The transformer neutral bus is the only point in the system where the neutral and ground must be connected.

Identify problems

Excessive ground current and ground loops can cause equipment malfunctions, misreading of instruments, and safety problems. There are two possible causes of excessive ground current: Poor NT bonding which can manifest itself in subpanels, receptacles, or equipment; and “insulated” ground rods. The subpanel NT connections create a parallel current path allowing normal return current to flow through the drain wire. This creates a situation where safety ground would become the only return path if the neutral were to open. Dangerous voltages could develop if the return path is high impedance.

Separate “isolated” earth rods are known to create two earth references at different potentials. This situation causes current to flow in ground loops to equalize the difference between potentials. This can cause intermittent system and equipment problems and pose safety and equipment risks.

Inspection of transformer grounding should be part of routine maintenance. Here are some things to check when checking transformer grounds:

• Verify the integrity of the NT link using a high quality ground impedance tester; high impedance NT connection can cause voltage fluctuations
• Check the integrity of the earth wire and its connection to the structural steel using a high quality earth impedance tester: fault currents return to the source through these connections, their impedance must be as low as possible
• Measure the ground current on the ground wire using a high quality clamp meter; a result greater than 1A suggests a problem

Grounding is essential to the safety and reliability of the installation. However, it can be difficult to understand the key concepts of proper grounding and the requirements of the NEC. Not leaving anything to chance. If in doubt, contact the relevant local authorities who will assist you.