BS EN 60255-121-2014 PDF

Full title and description

STB BS EN 60255-121:2014 — Measuring relays and protection equipment — Part 121: Functional requirements for distance protection. This standard specifies minimum functional and performance requirements, test methods and documentation practices for distance protection functions (impedance-plane based) used on transmission and distribution lines in effectively earthed three‑phase power systems.

Abstract

This document defines how distance protection functions (e.g. MHO, circular, quadrilateral/polygonal characteristics) shall be specified, tested and reported. It covers measurement characteristics, phase selection, directionality, inputs/outputs, timing and dynamic performance (transient overreach, operate time, SIR, behaviour under harmonics and CT/VT influences) and gives normative and informative test procedures and reporting requirements. It replaces IEC 60255-16 and follows the IEC 60255-1xx functional-standard structure.

General information

• Status: Published / Definitive (adopted as EN and published by national bodies).
• Publication date: 2014 (CENELEC approval 11 April 2014; EN publication in mid‑2014 — commonly listed as 31 July 2014 by some national publishers).
• Publisher: CENELEC / national standards bodies (published as BS EN by BSI and available as IEC 60255-121:2014).
• ICS / categories: 29.120.70 — Relays (Measuring relays and protection equipment).
• Edition / version: Edition 1.0 (2014) — IEC 60255-121:2014 adopted as EN 60255-121:2014 / BS EN 60255-121:2014.
• Number of pages: Approximately 148–152 pages (page count varies by national publication; typical listings show 148–152 pages).

Scope

The standard applies to distance protection functions whose operating characteristic is expressed on an impedance plane. It specifies minimum functional and performance requirements, measurement and directional criteria, starting and timing behaviour, influence of instrument transformers, dynamic performance tests (including transient overreach and operating time), and required documentation of type and performance tests. It does not provide full functional descriptions for ancillary features commonly found in digital relays (e.g. power swing blocking, out‑of‑step tripping, detailed VT supervision logic, or protection of series‑compensated lines), although their influence on distance protection behaviour is addressed.

Key topics and requirements

• Definition and specification of distance protection characteristics on the impedance plane (MHO, circular, quadrilateral/polygonal, offset types) and setting representation.
• Measurement and phase-selection requirements, directionality and fault discrimination.
• Requirements for current and voltage inputs, CT/VT influences (saturation, burden), and instrument transformer considerations.
• Starting logic, time delay characteristics, zone timers and behaviour for inrush, switch‑onto‑fault, reclose and loss‑of‑voltage conditions.
• Performance & accuracy tests: steady‑state accuracy, directional accuracy, transient overreach (TO), operate time, SIR and dynamic test methods.
• Test methodologies, ramping/impedance‑plane test procedures and requirements for type test reports and published performance data.

Typical use and users

Primary users are protection engineers, relay designers and manufacturers, transmission and distribution utilities, protection testing laboratories, commissioning teams and regulatory/standards personnel. The standard is used when specifying distance relay functionality, verifying relay performance during type testing and acceptance testing, and preparing published performance data for procurement and compliance.

Related standards

IEC 60255-1 (general requirements for measuring relays), other parts of the IEC 60255 series, the superseded IEC 60255-16 (canceled/replaced by -121), IEC 61850 (system and logical node mappings such as 21/PDIS), and IEEE/ANSI standards referenced for function numbering (e.g. C37.2). National adoptions appear as EN 60255-121:2014 and BS EN 60255-121:2014.

Keywords

Distance protection, distance relay, measuring relays, impedance plane, MHO characteristic, transient overreach (TO), SIR, CT saturation, VT/CVT, directional protection, zone timing, relay testing.

FAQ

Q: What is this standard?

A: It is EN/IEC 60255‑121:2014 — a functional standard that defines minimum functional and performance requirements and test methods for distance protection relays used on power transmission and distribution lines.

Q: What does it cover?

A: It covers specification of distance relay characteristics on the impedance plane, measurement and directional requirements, inputs/outputs, timing and dynamic performance testing (including transient overreach and CT/VT effects), and documentation of test results. It does not replace detailed descriptions of many ancillary relay features, but does address their influence on distance protection.

Q: Who typically uses it?

A: Protection engineers, relay manufacturers, utility protection and commissioning teams, testing laboratories and standards/technical committees use it for specifying, testing and documenting distance protection behaviour.

Q: Is it current or superseded?

A: IEC/EN 60255‑121:2014 is the 2014 edition and replaced IEC 60255‑16. Users should check national standards bodies or IEC/CENELEC for any later amendments, reconfirmation, or replacement after 2014 before relying on it for newest regulatory or product requirements.

Q: Is it part of a series?

A: Yes — it is part of the IEC 60255 series (Measuring relays and protection equipment). It follows the IEC structure for function‑specific parts (60255‑1xx). Related parts and general relay requirements (IEC 60255‑1 and other -1xx parts) should be consulted together with this part.

Q: What are the key keywords?

A: Distance protection, impedance plane, MHO, zone timing, CT/VT influence, transient overreach, SIR, directional relay, measuring relays.