A protective relay detects a fault. What does it actually do about it?

B. It closes (or opens) a small contact that trips a circuit breaker, and the breaker interrupts the fault

A. It opens the high-current circuit itself

In the standard device numbering, what is the difference between a 50 and a 51 element?

B. 50 is instantaneous overcurrent; 51 is time overcurrent (trips faster for bigger currents, following a curve)

A. 50 is for 50 Hz systems, 51 for 60 Hz

C. 50 is voltage, 51 is current

D. They are the same function from different makers

A differential (87) relay on a transformer compares the current going in with the current coming out. When does it trip?

B. When the two do not balance — current is going somewhere inside the protected zone it should not

A. Whenever the load current is high

D. Only on a ground fault outside the transformer

You need to disconnect a meter wired to a CT secondary while the primary is carrying load. What do you do first?

B. Short the CT secondary with its shorting block (or switch) before breaking the circuit to the meter

A. Open the secondary leads quickly

C. Nothing — CT secondaries are low voltage

D. Ground one lead and then open the other

What is the job of a PT (potential or voltage transformer) in a relaying scheme?

B. To step a high system voltage down to a safe, standard level the relay or meter can measure

A 27 element operates on undervoltage and a 59 on overvoltage. Where might a 27 be used?

B. To drop a motor or start a transfer to standby power when the supply voltage sags or is lost

D. To detect a ground fault on an ungrounded system only

A lockout relay (86) has tripped after a transformer differential operation. Why is it designed so that it must be reset by hand?

B. So the equipment cannot be re-energized automatically onto an internal fault — someone must investigate first

C. Because it has no reset coil

Fourth year Lesson 10 of 12 · about 14 min

Protective relaying concepts

What you’ll learn

How a relay, its CTs and PTs, and a breaker work together: sense, decide, trip.
What the common device numbers on a single-line mean — 50, 51, 87, 27, 59, 86, 52.
How a differential relay tells an internal fault from a heavy through-fault.
Why a CT secondary is never opened under load, and why lockout relays are reset by hand.

Before this: Overcurrent protection and coordination · Available fault current

A protective relay watches a scaled-down copy of the current and voltage, decides whether something is wrong, and closes a contact that trips a breaker. It never interrupts the fault itself. Sensing is done by instrument transformers, deciding by the relay, and interrupting by the breaker — and a failure in any one of the three means no protection.

The eyes: CTs and PTs

A relay cannot connect to thousands of amps or thousands of volts. A current transformer (CT) puts out a small current in proportion to the primary current — a 600:5 CT gives 5 A when 600 A flows. A potential transformer (PT, or VT) steps the voltage down to a standard low value. The relay then works on those copies.

Never open a CT secondary with current in the primary. A CT is built to drive its current into a near short. Open the loop and it tries anyway — the secondary voltage climbs until something breaks down: the insulation, an arc, or you. Short the secondary with its shorting block before you disconnect anything on it. The transformer basics page explains why.

The brain: device numbers

Number	Function	What it is watching for
50	Instantaneous overcurrent	A heavy fault close by — trip with no intentional delay
51	Time overcurrent	Overcurrent that follows an inverse curve: more current, faster trip
50N / 51N / 51G	Ground (residual or neutral) overcurrent	Current returning by a path other than the phase conductors
87	Differential	Current in ≠ current out of a protected zone
27 / 59	Under / overvoltage	Loss or sag of supply; voltage too high
86	Lockout relay	Holds breakers tripped until reset by hand
52	AC circuit breaker	The muscle — opens the power circuit

How a differential relay thinks

The differential idea. It does not care how big the current is, only whether what goes into the zone comes back out. That makes it fast and selective — it ignores faults outside its zone completely.

Real transformer differential relays have to allow for the turns ratio, the 30° shift through a delta-wye bank and the inrush when the transformer is energized, which is why their settings and CT connections are an engineering job. The idea underneath is still “in should equal out”.

Worked: what the relay sees through a CT

A feeder has 600:5 CTs. Ratio = 600 ÷ 5 = 120.
Load current 360 A: the relay sees 360 ÷ 120 = 3 A.
A fault of 9,600 A: the relay sees 9,600 ÷ 120 = 80 A — for the few cycles until it trips. That is why CT secondary wiring and test blocks are built heavy.
A relay set (by the engineer) to pick up at 4 A secondary is picking up at 4 × 120 = 480 A primary.

The muscle: the trip circuit

The relay’s trip contact energizes the breaker’s trip coil, usually from a station battery so it works when the AC is collapsing. A lockout relay (86) often sits between them: once it operates it holds every breaker it controls open until someone resets it by hand. That is deliberate — after an internal fault, re-energizing automatically is the wrong answer. Test switches let technicians isolate the trip circuit and the CTs safely while they test.

On the job

Tips

Learn to read the device numbers on a single-line; they tell you what protects what.
Short CT secondaries at the test block before you touch the wiring downstream — every time.
Check the trip circuit is healthy (many panels have a trip-circuit monitor light). A relay with no trip path is decoration.
Record relay targets (the flags or event log) before anyone resets anything; they are the evidence.

Common mistakes

Opening a CT circuit under load.
Resetting an 86 lockout without finding out why it operated.
Rolling a CT’s polarity (swapping the secondary leads) — a differential scheme will then trip on load.
Changing relay settings to stop nuisance trips instead of finding the cause.
Confusing the 52 (the breaker) with the relay that tells it to open.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.A protective relay detects a fault. What does it actually do about it?
2.In the standard device numbering, what is the difference between a 50 and a 51 element?
3.A differential (87) relay on a transformer compares the current going in with the current coming out. When does it trip?
4.You need to disconnect a meter wired to a CT secondary while the primary is carrying load. What do you do first?
5.What is the job of a PT (potential or voltage transformer) in a relaying scheme?
6.A 27 element operates on undervoltage and a 59 on overvoltage. Where might a 27 be used?
7.A lockout relay (86) has tripped after a transformer differential operation. Why is it designed so that it must be reset by hand?

Answers

B. It closes (or opens) a small contact that trips a circuit breaker, and the breaker interrupts the fault — A relay is the brain, not the muscle. It watches current and voltage through CTs and PTs, decides, and operates a trip contact. The breaker is what opens the power circuit — which is why a relay with a dead trip circuit protects nothing.
B. 50 is instantaneous overcurrent; 51 is time overcurrent (trips faster for bigger currents, following a curve) — 50 trips with no intentional delay above its pickup — for heavy faults close by. 51 follows an inverse time curve, so modest overloads take longer and big faults trip quickly. Most feeder relays have both.
B. When the two do not balance — current is going somewhere inside the protected zone it should not — For a fault outside the zone, the same current flows in and out, the CT currents cancel and the relay stays put however large the current. A fault inside the zone makes in ≠ out, and the difference operates the relay. That makes it fast and selective.
B. Short the CT secondary with its shorting block (or switch) before breaking the circuit to the meter — An open CT secondary with primary current flowing can develop a very high voltage — dangerous to you and to the insulation. Always short the secondary first, then work on the downstream device.
B. To step a high system voltage down to a safe, standard level the relay or meter can measure — Relays and meters cannot connect directly to medium or high voltage. A PT gives them a scaled-down copy of the voltage, just as a CT gives them a scaled-down copy of the current.
B. To drop a motor or start a transfer to standby power when the supply voltage sags or is lost — Undervoltage relays catch loss or sag of supply — used to drop motors so they do not all restart together, or to signal a transfer switch to start the generator.
B. So the equipment cannot be re-energized automatically onto an internal fault — someone must investigate first — An 86 trips and holds the breakers open until a person resets it. An internal transformer fault re-energized automatically could turn damage into destruction. The lockout forces the question “why did it trip?” to be answered first.

Questions people ask

Why use relays when breakers already have trip units?

Low-voltage breakers do have built-in trip units, and for most building work that is all there is. Above that — medium-voltage switchgear, large transformers, generators, utility ties — the breaker is just a switch that can interrupt a fault, and separate relays fed by CTs and PTs decide when to open it. Relays can do things a trip unit cannot: compare currents in two places, look at voltage and frequency, and coordinate across a whole system.

What do the device numbers mean?

They are a standard shorthand (IEEE/ANSI C37.2) used on single-lines and relay panels worldwide: 50 instantaneous overcurrent, 51 time overcurrent, 50/51N or 51G for the ground versions, 87 differential, 27 undervoltage, 59 overvoltage, 81 frequency, 86 lockout, 52 the circuit breaker itself. Seeing “51” on a drawing tells you the function, whatever make of relay is in the panel.

Who sets the relays?

The settings come from a protection engineer’s coordination study. Electricians and relay technicians install, wire, test and put those settings in; they do not choose them. Changing a relay setting because it “keeps tripping” is how coordination gets destroyed.

Transformer basics — the CT warning, in full
Overcurrent protection and coordination — time-current curves
Available fault current
Reading commercial and industrial prints — device numbers live on single-lines
Emergency and standby power

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