Protection Relays | Bishop Engineering

Quick pick (TL;DR)

Feeder → overcurrent 50/51 + earth fault 50N/51N.
Motor → add thermal 49, unbalance 46, locked-rotor 48, restart 66.
Transformer/generator → add differential 87.
Set pickup just above full load, choose an IDMT curve (IEC SI is the default), and grade each upstream relay by ≥ 0.3 s so downstream clears first.

Step 1 — Functions by equipment

Equipment	Core functions	ANSI
Feeder	OC + EF	50/51, 50N/51N
Motor	+ thermal etc.	49, 46, 48, 66
Transformer	+ diff	87T, 64REF
Generator	+ diff etc.	87G, 32, 40
Busbar	Bus diff	87B

Step 2 — Grading example

IDMT grading (IEC SI)

Given full-load current = 166 A , CT = 200/1 fault current = 6000 A Step 1 — Pickup Is = 1.05–1.2 × FLC ≈ 175 A plug setting = 175 / 200 = 0.875 ≈ 0.9 (90%) Step 2 — Operate time PSM = 6000 / 175 = 34 t = TMS × 0.14 / (PSM^0.02 − 1) t = 0.20 × 0.14 / (34^0.02 − 1) t = 0.028 / 0.0733 = 0.38 s Step 3 — Grade upstream upstream t = 0.38 + 0.30 = 0.68 s

IDMT gives a definite time per fault level; grade ≥ 0.3 s so downstream clears first.

Step 3 — CTs & communication

Size CT above FLC; check knee-point.
PX/5P for protection, 0.5 for metering.
IEC 61850 (GOOSE); Modbus/DNP3 legacy.

IDMT relay time calculator

IEC inverse-time operate time

t = TMS·k/(PSM^α − 1), PSM = I_fault / I_set.

Fault current (A)

Pickup Is (A)

TMS

Curve

Grading margin (s)

Pickup ~1.05–1.2× FLC. Check CT doesn't saturate at fault current.

Common mistakes

Pickup below full load.
No grading margin.
CT saturation at fault current.

Requisition & datasheet

Equipment, voltage, current, CT ratio.
Functions, pickup, TMS, curve, comms.

Frequently asked questions

What should I know about Protection Relays & Settings?

Feeder → overcurrent 50/51 + earth fault 50N/51N. Motor → add thermal 49, unbalance 46, locked-rotor 48, restart 66. Transformer/generator → add differential 87. Set pickup just above full load, choose an IDMT curve (IEC SI is the default), and grade each upstream relay by ≥ 0.3 s so downstream clears first.

Which standards or codes apply?

IEC 60255 (relays), IEEE C37.2 (ANSI device numbers), IEC 61850 (substation comms), IEC 60044 / 61869 (instrument transformers).

Standards — IEC 60255; IEEE C37.2; IEC 61850; IEC 61869.

Protection Relays & Settings