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Ground-fault & protection relaying

While ground-fault protective schemes may be elaborately developed, depending on the ingenuity of the relaying engineer, nearly all schemes in common practice are based on one or more of the methods of ground-fault detection discussed in this article.

4 essential ground-fault protective schemes you should know about
4 essential ground-fault protective schemes you should know about

Distribution circuits that are solidly grounded or grounded through low impedance require fast clearing of ground faults. This need for speed is especially true in low-voltage grounded wye circuits that are connected to busways or long runs of metallic conduit.

The problem involves sensitivity in detecting low ground-fault currents as well as coordination between main and feeder circuit protective devices.

This article is primarily directed to ground-fault protection of major portions of circuits and equipment.

The discussions relate to various forms of ground-fault protection to prevent excessive damage to electrical equipment with current sensitivity in the order of amperes to hundreds of amperes.

Now, let’s talk a little bit about the these methods of ground-fault detection:

  1. Residually connected overcurrent relays
  2. Core balance sensing of feeder conductors
  3. Detection of ground-return current in the equipment grounding circuit
  4. Differential relaying

1. Residual connection

A residually connected ground relay is widely used to protect medium-voltage systems. The actual ground current is measured by CTs that are interconnected in such a way that the ground relay responds to a current proportional to the ground-fault current.

This scheme, using individual relays and CTs, is not often applied to low-voltage systems. However, low-voltage systems are available with three CTs built into them and connected residually with the solid-state trip devices to provide ground-fault protection.

The term residual in common usage is normally reserved for three-phase system connections and seldom applied to single-phase or multiple-signal mixing.

The basic residual scheme is shown in Figure 1. Each phase relay is connected in the output circuit of its respective CT, while a ground relay connected in the common or residual circuit measures the ground-fault current.

Residually connected ground relay
Figure 1 – Residually connected ground relay

In three-phase three-wire systems, no current flows in the residual leg under normal conditions because the resultant current of the three CTs is zero.

This situation is true for phase-to-phase short circuits also. When a ground-fault occurs, the short-circuit current, returning through earth, bypasses the phase conductors and their CTs, and the resultant current flows in the residual leg and operates its relay.

On four-wire circuits, a fourth CT should be connected in the neutral circuit as shown in the dashed portion of the current. The neutral conductor carries both 60 Hz single-phase load unbalance current as well as zero-sequence harmonic currents caused by the nonlinear inductance of single-phase loads, such as fluorescent lighting.

Without the neutral-conductor CT, the current in that conductor would appear to the ground relay as ground-fault current, and the ground relay would have to be desensitized sufficiently to prevent opening under unbalanced load conditions.

The selectivity of residually connected relays is determined by the CT ratio and the relay pickup setting. The CT ratio must be high enough for normal load circuits.

Also to be considered are the unbalanced primary currents in each phase, the sum of which may contain sufficient asymmetrical current to cause a trip during a motor start.

For this reason, sensitive ground-fault protection schemes do not use instantaneous trips when a residually connected circuit is involved. If ground-fault protection with greater sensitivity is needed, the core-balance method should be considered (see next paragraphs).

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2. Core balance

The core-balance method is based on primary current phasor addition or flux summation. The remaining zero-sequence component, if any, is then transformed to the secondary.

The core-balance CT or sensor is the basis of several low-voltage ground-fault protective systems.

The core-balance CT is frequently called a zero-sequence sensor or window CT, but the term core balance is preferable because it more specifically describes the function of the CT.

The principle of the core-balance CT circuit is shown in Figure 2.

Core-balance CT encircling all phase and neutral conductors
Figure 2 – Core-balance CT encircling all phase and neutral conductors

All load-carrying conductors pass through the same opening in the CT and are surrounded by the same magnetic core.

Core-balance CTs are available in several convenient shapes and sizes, including rectangular designs for use over busbars.

This method can be more sensitive than the residual method because the sensor rating is large enough for the possible imbalance, not for the individual conductor load current.

Core-balance CTs various sizes
Core-balance CTs various sizes (photo credit: newelec.co.za)

Under normal conditions (i.e., balanced, unbalanced, or single-phase load currents or short circuits not involving ground – if all conductors are properly enclosed), all current flows out and returns through the CT. The net flux produced in the CT core is zero, and no current flows in the ground relay.

When a ground fault occurs, the ground-fault current returns through the equipment grounding circuit conductor (and possibly other ground paths) and bypasses the CT. The flux produced in the CT core is proportional to the ground-fault current, and a proportional current flows from the CT secondary to the relay circuit.

Relays connected to core-balance CTs can be made quite sensitive. However, care is necessary to prevent false opening from unbalanced inrush currents that may saturate the CT core or through faults not involving ground.

If only phase conductors are enclosed and neutral current is not zero, the transformed current is proportional to the load zero-sequence or neutral current.

Systems with grounded conductors, such as cable shielding, should have the CT surround only the phase and neutral conductors, if applicable, and not the grounded conductor.

By properly matching the CT and relay, ground-fault detection can be made as sensitive as the application requires.

The speed of the relay limits damage and may be adjustable (for current or time, or both) in order to obtain selectivity. Many ground protective systems now have solid-state relays specially designed to operate with core-balance CTs. The relays in turn open the circuit protective device. Power circuit breakers, MCCBs with shunt trips, or electrically operated fused switches can be used.

The last category includes service protectors, which use circuit breaker contacts and mechanisms, but depend on current-limiting fuses to interrupt the high available short-circuit currents.

Fused contactors and combination motor starters may be used where the device-interrupting capability equals or exceeds the available ground-fault current.

Physical installation of a core-balanced CT for shielded cable termination
Figure 3 – Physical installation of a core-balanced CT for shielded cable termination

Figure 3 shows a typical termination of a medium-voltage shielded cable. After the cable is pulled up through the core-balance CT, the cable jacket is removed to expose the shielding tape or braid. Jumpering the shields together, the connection to the ground is made after this shield lead is brought back through the CT.

This precaution would have been necessary only if the shield had been pulled through the CT.

Between multiple shield ground connections on a single conductor cable, a potential exists that drives a circulating current, often of such a magnitude as to require derating of the cable ampacity.

When applying the core-balance CT, the effects of this circulating current should be subtracted from the measuring circuit.

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3. Ground return

Ground-return relaying is illustrated in Figure 4. The ground-fault current returns through the CT in the neutral-bus to ground-bus connection.

For feeder circuits, an insulating segment may be introduced in busway or conduit, as shown in Figure 4, and a bonding jumper connected across the insulator to carry the ground-fault current. A CT enclosing this jumper then detects a ground fault.

This method is not recommended for feeder circuits due to the likelihood of multiple ground-current return paths and the difficulty of maintaining an insulated joint.

Ground-return relay scheme
Figure 4 – Ground-return relay scheme

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4. Ground differential

A generic term, ground differential is used for a variety of schemes that utilize phasor or algebraic subtraction or addition of signals. The currents may be produced by any of the methods discussed in sections 1 through 3.

Figure 5, for example, shows a ground differential protection for single-phase center-tapped loads and is similar to the residual method.

Differential protection center-tapped loads
Figure 5 – Differential protection center-tapped loads

One core-balance sensor could detect ground faults in a plurality of loads (see Figure 6). Ground-differential relaying is effective for main bus protection because it has inherent selectivity.

Differential protection multiple loads
Figure 6 – Differential protection multiple loads

With the differential scheme (see Figure 7), core-balance CTs are installed on each of the outgoing feeders and another lower ratio CT is placed in the transformer neutral connection to ground.

This arrangement can be made sensitive to low ground-fault currents without false opening for ground faults beyond the feeder CTs.

All CTs should be carefully matched to prevent improper opening for high-magnitude faults occurring outside the differential zone.

Ground-differential scheme bus and transformer
Figure 7 – Ground-differential scheme bus and transformer

Bus-differential protection protects only the zone between CTs and does not provide backup protection for feeder faults.

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Circuit sensitivity

Factors that affect the sensitivity of ground-fault sensing include the following:

  1. Circuit-charging current drawn by surge arrestors, shielded cables, and motor windings
  2. The number of coordination steps between the branch circuit and the supply
  3. Primary rating and accuracy of the largest CT used to supply residually connected relays in the coordination
  4. How well the CTs used for residually connected relays are matched
  5. Burden on the CTs, in particular the burden of the residually connected relay (Solid-state and some induction disk relays have burdens of 40 VA for a 0.1 A tap.)
  6. Maximum through-fault current and its effect upon the CTs, with selected relays for phase and residual connections
  7. Fault contact resistance
  8. Location of conductors within core-balance transformers

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Reference // IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Std 242)

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

2 Comments


  1. H.Peter
    Oct 29, 2018

    I real need this slides for my exam can u e mail me this please they are going to help me if get them they are nice real


  2. Aravind Kumar
    Oct 29, 2018

    Hi,
    One of our site we had a different issue, we disconnected the main source switching of the LT switch gear at HT yard and our DG has started automatically, but in the panel room got the fluctuations immediately, so we disconnected the entire Ring main unit and the power was stabled.
    What could be the reason? Please help me out

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