Protection Relays & Switchgear

The guide on transformer differential protection (87T) covers protection against internal short circuits. This article covers a completely different failure mechanism that 87T does not detect: core saturation caused by an excessive voltage-to-frequency ratio, and the ANSI 24 V/Hz protection that monitors it with an inverse-time characteristic.

The guide on circuit breaker settings covers the LSI(G) curve of a low-voltage power circuit breaker. This article covers the related, but not identical, principle used with medium-voltage protection relays: the inverse definite minimum time (IDMT) characteristic under ANSI 50 (instantaneous overcurrent) and ANSI 51 (time overcurrent), and why the choice of curve type determines selectivity between relays in series.

The guide on distance protection (21) covers the impedance zones of a single relay. This article covers how two relays at either end of a line use a communication channel — ANSI code 85 — to exchange information so a fault over the full line length can be tripped instantaneously, and the difference between permissive and blocking teleprotection schemes.

The Buchholz relay, mounted in the pipe between the main tank and the conservator vessel of an oil-filled power transformer, detects an incipient fault via slow gas accumulation (alarm stage) and a severe fault via a sudden oil surge (trip stage) — two fully separate protection mechanisms that operate independently of the electrical differential and overcurrent protection. Why an alarm signal must never be ignored without gas analysis, and why hermetically sealed transformers without a conservator have no Buchholz relay.

The guide on transformer differential protection (87T) covers percentage-differential protection against phase-to-phase faults; restricted earth fault (REF, ANSI code 64N or 87N) is a separate, high-impedance differential scheme sensitive only to earth faults within a strictly bounded zone (usually one winding plus its associated cable), and it can detect an earth fault close to the star point that ordinary differential protection can miss because the fault current there stays low due to the low effective turns ratio. Why the high-impedance principle with a stabilizing resistor lets a REF relay operate for an internal fault while remaining stable during a heavy external fault, and why the knee-point voltage of the current transformers used plays a critical role.

A percentage differential relay (ANSI code 87T) compares the current entering a transformer with the current leaving it, via secondary current transformers on both sides; in a healthy transformer the difference is virtually zero, in an internal fault it is not. Why this difference is always assessed against a percentage of the through-flowing (restraint) current rather than a fixed threshold, why the CT ratios and the transformer's vector group must be compensated electronically or with interposing transformers, why second-harmonic blocking distinguishes inrush current from a real fault, and why this relay can miss a low-energy, slowly developing fault that the Buchholz relay does detect.

Besides the Buchholz relay, an oil-filled power transformer typically also has thermal monitoring: an oil temperature indicator (OTI) measures the temperature of the top oil layer directly with a sensor in a pocket, while a winding temperature indicator (WTI) simulates the winding hot-spot temperature using the 'thermal image' principle — a heating element fed by the load current, added on top of the OTI measurement, because a sensor cannot be placed directly inside the winding itself. Why the OTI responds slowly (hours) and the WTI much faster (minutes), and why this thermal monitoring signals a different type of fault than the Buchholz relay.

IEC 60947-2: the adjustable trip parameters of an electronic circuit-breaker trip unit (Ir/tr for long-time/thermal protection, Isd/tsd for short-time/magnetic protection, Ii for instantaneous protection and Ig/tg for ground-fault protection) — and why an incorrectly set curve undoes selectivity between two circuit-breakers even when both breakers themselves are correctly chosen.

Besides the well-known Icu (ultimate breaking capacity) and Ics (service breaking capacity), IEC 60947-2 defines two separate short-circuit characteristics for circuit breakers: Icw, the RMS short-time withstand current that a breaker with an intentional time delay (for selectivity) must withstand without damage during that delay (typically 0.05-1 s), and Icm, the peak-value making capacity with which the breaker may close onto an already-present short circuit without the contacts welding shut due to electrodynamic forces. Why a breaker with an ample Icu can still be unsuitable for a selective network if its Icw falls short.

IEC 60947-2 (MCCB) versus IEC 60898-1 (MCB): why a moulded-case circuit breaker cannot simply be replaced by a miniature circuit breaker — differences in target user group, current range, and the Icu/Ics versus Icn rating of short-circuit withstand capability.

The guide on LSI(G) settings covers how the short-time delay (tsd) of an upstream circuit-breaker is typically set longer than the total trip time of a downstream breaker, to achieve selectivity — with the downside that the upstream breaker waits just as long for any fault within its own zone, even one occurring right beneath it. Zone selective interlocking (ZSI) solves this with a restraint signal between breakers: a downstream breaker that sees a fault sends a signal upstream that forces the upstream breaker to hold its normal delay; if that signal is absent, the upstream breaker trips almost instantly, significantly reducing arc-flash energy for a fault close to that breaker.

IEC 60947-4-1 defines the trip class of a motor overload relay through one standardised test: at 7.2x the set current from cold, Class 10A must trip within 2-10s, Class 10 within 4-10s, Class 20 within 6-20s and Class 30 within 9-30s — the correct class depends on the actual starting time of the driven load, not on the motor alone.

IEC 61439-2 distinguishes four forms of internal separation within a low-voltage switchgear assembly: Form 1 (no separation between busbars, terminals and functional units), Form 2 (busbars separated from functional units; 2a without, 2b with separated terminals), Form 3 (functional units separated from each other and from the busbars; 3a without, 3b with separated terminals), and Form 4 (functional units, including their terminals, fully separated; 4a terminals within the same compartment as the unit, 4b terminals in a separate compartment). Why a higher form is mainly about working safely on one unit while the rest of the assembly stays live, and why this is a property of the assembly itself, not of the surrounding installation.

The guides on LSI(G) settings and zone-selective interlocking (ZSI) assume that a circuit breaker actually opens once it receives a trip command. This article covers what happens when that assumption fails: breaker failure protection (ANSI code 50BF) monitors, after every trip command, whether the current through the breaker actually disappears, and if not, sends a back-up trip command within a short, fixed time to every surrounding breaker that can also interrupt the fault. Why this time delay is a fundamental trade-off between extra tripping time during a fault and the risk of a failed breaker letting the fault persist indefinitely.

The guide on directional overcurrent protection (67) and the guide on generator field-failure protection (40) cover protections that respond to current direction and terminal impedance respectively. This article covers distance protection (ANSI 21), which calculates the impedance to a fault to determine the fault distance, using a staggered zone 1/2/3 scheme of reach and time delay, and why this function is relatively insensitive to variations in available fault current.

The guide on generator grid connection (WKK) and the guide on reverse-power protection (32) cover situations where a generator runs in parallel with the grid. This article covers the protection function that supervises the act of paralleling itself: synchronizing check (ANSI 25), which checks voltage difference, frequency difference (slip), and phase-angle difference between two sources before a breaker is allowed to close, and why an incorrect paralleling operation can cause major mechanical and electrical damage.

The guides on reverse power protection (32) and directional overcurrent protection (67) cover protection functions that respond to current. This article covers a protection function that responds directly to terminal voltage itself: undervoltage protection (ANSI 27) and overvoltage protection (ANSI 59), why both are needed alongside ordinary overcurrent protection, and the typical setting limits (generally 80-90% Un for 27, 110-130% Un for 59) with their associated time delay.

The guide on paralleling and synchronizing standby generators covers how a generator is brought onto the grid in synchronism, but not what happens if that generator's drive then fails while it stays electrically connected. This article covers reverse power protection (ANSI code 32): why a generator without drive power is driven as a motor by the grid instead of the other way around, why this is more quickly damaging for some drives than others, and why the setting sensitivity is typically only a few percent of rated power.

The guides on under-/overvoltage protection (27/59) and reverse-power protection (32) cover generator protections that react to terminal voltage or the direction of active power. This article covers a third category: field-failure protection (ANSI 40), which recognizes (partial) loss of excitation of a synchronous generator via an offset-mho impedance relay monitoring terminal impedance, and why this function must be coordinated with the generator's capability curve.

The guide on motor overload classes (10/20/30) covers thermal overload protection based on total current. This article covers an additional function that reacts specifically to unbalance: negative-sequence protection (ANSI 46), which monitors the negative-sequence current (I2) of a motor or generator, why a small unbalance causes disproportionate rotor heating, and how this connects to the guide on three-phase voltage unbalance.

The guides on busbar differential protection (87B) and breaker failure protection (50BF) assume a network with a clear, fixed current direction per feeder. This article covers what happens once that no longer holds — for example on a ring network, a double-fed busbar, or distributed generation — and how directional overcurrent protection (ANSI 67) uses a polarizing voltage to determine current direction so it only operates for a fault in its own, monitored direction.

The guide on directional overcurrent protection (67) covers how a fault is selectively recognized. This article covers what happens next on an overhead medium-voltage line: automatic reclosing (ANSI 79, recloser), which is based on the fact that most faults on an overhead line are transient, and why the dead time between tripping and reclosing must be chosen carefully.

The guide on anti-islanding protection for PV inverters briefly mentions ROCOF as one of the detection methods within NEN-EN 50549-1. This article covers the underlying protection function in more detail: over- and underfrequency protection (ANSI 81O/81U) and the rate of change of frequency (ROCOF, df/dt, ANSI 81R), how these are used for both ordinary load shedding and islanding detection for distributed generation, and why voltage supervision is needed to prevent false tripping.

The guides on transformer, motor, and busbar differential protection (87T/87M/87B) cover how an internal fault is detected. This article covers what happens next: the lockout relay (ANSI 86, master trip relay), which acts as an intermediary between multiple protection relays and the breaker, latches the trip command, and requires a deliberate manual reset before the installation can be put back into service.

The guide on transformer differential protection (87T) and the guide on REF protection (64N/87N) cover differential principles applied to a transformer winding. This article covers the same differential principle applied to a completely different part of the installation: the busbar of a switchboard itself. Why a fault on the busbar is often cleared slowly by ordinary feeder protection, how busbar differential protection (ANSI 87B) monitors the sum of all currents entering and leaving the busbar zone, and the difference between a high-impedance and a low-impedance implementation of this principle.

The guide on transformer differential protection (87T) covers the percentage-differential principle for a power transformer. This article covers the same protection philosophy applied to a large or critical motor: motor differential protection (ANSI 87M), the difference between the self-balancing (core-balance) and the six-CT current-summing method, and why this function is faster and more sensitive than the ordinary thermal overload protection from the motor protection class guide.

A secondary injection test feeds a test signal directly into the protection relay or electronic trip unit, and only verifies the relay and its wiring — not the current transformer (CT) or the actual physical tripping of the circuit-breaker itself. A primary injection test drives a real, high test current through the entire primary current path (CT primary winding, CT secondary winding, relay/trip unit and the breaker itself) and is therefore the only method that demonstrates the complete protection chain including wiring errors between CT and relay — but it requires taking the installation out of service and specialised, heavy test equipment.

IEC 62271-100: for a routine contact resistance measurement of a circuit breaker (using at least 50 A DC test current via the 4-wire Kelvin method), the measured resistance must not exceed 1.2 times the reference value Ru from the type test — an increase beyond that points to contact wear or contamination. Why this routine acceptance limit differs from the general 'compare against similar connections' approach used for cable and busbar joints, and why a trending increase across successive periodic measurements matters as much as the absolute value.