3.4 Grounding Systems & Impedance Grounding

Grounded vs. Grounding: The NEC Distinction

Quick Answer: The grounded conductor is the current-carrying conductor bonded to ground at the service (usually the neutral). The grounding conductor (equipment grounding conductor, EGC) bonds non-current-carrying metal parts together and to ground; it carries current only during a fault.

NEC Article 100 and 250 make this distinction because the two conductors serve different purposes:

• Grounded conductor: carries normal load current (unbalanced neutral current in a 3φ 4-wire system) and is bonded to the grounding electrode system at the service disconnect (main bonding jumper). It is a current-carrying conductor that happens to be grounded.
• Equipment grounding conductor (EGC): bonds equipment enclosures, raceways, and cable trays together and to the system grounding point. It carries fault current only when a phase-to-ground fault occurs, and its low impedance is what allows the overcurrent device to trip quickly.

On a 480Y/277 V service, the neutral is the grounded conductor; the green/bare bond wire in the conduit is the EGC. Mixing these up on the exam is a guaranteed wrong answer.

System Grounding Types

Why Limit Ground-Fault Current?

A solidly grounded 13.8 kV system can deliver 20,000+ A into a ground fault. That current does three damaging things:

1. Arc burn at the fault point: melts cable, switches, and bus at the arc location.
2. Mechanical stress on the equipment: the magnetic forces from high fault current can deform bus bars and damage transformer windings.
3. Thermal damage to the neutral resistor/reactor and winding: sustained ground-fault current overheats the iron and copper.

Impedance grounding inserts a resistor or reactor between the transformer neutral and ground so the ground-fault current is predictable and limited. Industrial 4.16 kV and 13.8 kV systems often use low-resistance grounding (~400 A) to allow selective relaying while reducing damage. High-resistance grounding (~5-10 A) is used where continuity of service is critical (refineries, hospitals, data centers) — the system alarms, the operator locates the fault, and the process keeps running until a planned shutdown.

The Ground-Fault Current Path and the EGC

On a solidly grounded system, a ground fault on a piece of equipment sends current from the source, through the phase conductor, through the fault to the equipment enclosure, through the EGC back to the service neutral-ground bond, and through the grounded neutral to the source. The EGC's impedance determines how much current flows and how fast the breaker trips. A high-impedance EGC (loose connection, undersized wire, corroded bond) means low fault current and a slow (or no) trip — exactly the condition that starts fires and shocks workers. This is why NETA testing includes the ground-fault circuit impedance test and visual inspection of bonding and grounding connections.

Ground-Fault Detection

Three common detection schemes:

• Residual (wye-connected CTs): the three phase CT secondaries are summed. In a balanced system the sum is zero; a ground fault produces residual current. Simple and common on 51N/50G relays.
• Zero-sequence (core-balance) CT: all three phase conductors (and the neutral if present) pass through one CT. Any imbalance indicates ground-fault current. Sensitive — used for GFPE and high-resistance grounded system alarms.
• Neutral CT or shunt: a CT on the neutral-to-ground bond directly measures ground-fault current. Used on resistance-grounded systems where the fault current returns through the resistor.

On a high-resistance grounded system the neutral resistor is sized so the available ground-fault current equals or slightly exceeds the system's total charging current (so the zero-sequence capacitive current is canceled and the alarm is meaningful). A 13.8 kV system with 15 A charging current might use a 15-20 A neutral resistor.

The Open-CT Hazard Rule (Critical Safety)

A current transformer is a constant-current source (within its accuracy range) on the secondary side. The secondary current is fixed by the primary current and the turns ratio: Isec = Iprim / N. The voltage developed across the secondary is Vsec = Isec × Zburden. If you remove the burden (open the secondary), Zburden approaches infinity and Vsec rises until the core saturates — but saturation still leaves a high, dangerous peak voltage that can:

• Break down the CT's secondary insulation.
• Produce a lethal shock hazard at the open terminals.
• Drive the CT into saturation that ruins its accuracy until demagnetized.

Rule: never open the secondary of an energized CT. If you must remove a relay or meter, short the CT secondary first with a CT shorting block or jumper, then disconnect the burden. Many test sets and switchgear CT terminal blocks include built-in shorting bars that engage before the burden lifts. NETA field work routinely involves isolating CTs for injection testing — the shorting sequence is non-negotiable and a frequent exam question.

The same rule does not apply to potential transformers (PTs) — a PT secondary can be opened (like any voltage source) but must never be shorted, and PT secondaries should be fused and grounded.

Exam Trap

A common trap confuses 'solidly grounded' with 'low-impedance grounded.' Solid grounding means the neutral is bonded to ground with no intentional impedance — the fault current is limited only by the source and transformer impedances. Low-resistance and high-resistance grounding add a deliberate resistor in the neutral. If a question says 'resistance-grounded,' do not treat it as solidly grounded for fault calculations — the resistor dominates Z0.