EMC AT CERN GUIDELINES

EMC AT CERN

GUIDELINES

J. GASCON

DGS/SEE

EDMS n° 1287214 EMC at CERN J. Gascon

Introduction

Why to earth CERN installations?

To reduce risk in case of electrical hazards (safety)

To decrease instrumentation & control errors (EMC)

To minimise unnecessary downtime and impacted areas

To moderate cascade equipment failures

To avoid damages due to lightning strikes


Introduction

Functions of an earthing system

• Personnel safety: earth conductors evacuates lightning and short-circuit

currents and protect people from electrical hazards

• Equipment safety: earthing system prevent damage to the equipment caused by

lightning strike and short-circuit currents avoiding fire, flashovers, etc.

• EMC compatibility: an effective earthing system ensure equipment

equipotentiality*. A correct equipotentiality is necessary for EMC at all

frequencies.

* Equipotentiality: state when conductive parts are at a substantially equal electric potential (IEC 60050 - 195-01-09)


CERN: a mesh Common Bonding Network


Impedance of a meshed network

For a conductor, the impedance between the ends increases as its length.

For a 2D grid, the impedance between 2 points does not depend on their distance.

For a 3D grid, the impedance between 2 points decreases with the size of the structure !…

Inductance : L

(L ≈ 1 µH/m)

LAB ≈ L

LCD ≤ L

A

B

C

D


EMC Equipotential Bonding

• Process to make low impedance path for the flow of electrical

currents established between two conductive parts

• Equipotentiality reduces differences in potential between points

within signal reference, shielding, and lightning protection system

of electronics.

• Poor bonds, lead to hazardous situations and interferences.

• A bonding resistance of 1 milliohm or less is considered a high

quality bond.


A star grounding scheme creates

a large common impedance

between interconnected

equipments

P. Supply

500 mA

Pulses

at 500 kHz

Variable

Frequency

Drive

Control

cabinet

50 V

Isolated link cable

30 m long

PE

Z ≈ 100 Ω

at 500 kHz

500 mA

100 Ω

500 mA

Pulses

at 500 kHz

Variable

Frequency

Drive

Control

cabinet

< 1 V

250 mA 250 mA

< 1 m

Bonding


Equipment

#1

5 kV

Isolated link

cable

50 m2

Ground

loop

Lightning magnetic

field (= 80 Am-1µs-1)

Equipment

#2

A star grounding scheme creates

a large area ground loop

between interconnected

equipments

< 50 V

< .5 m2 loop

80 Am-1µs-1

Equipment

#1

Equipment

#2


Regulations for EMC

• EMC 2004/108/CE: Directive of the European Parliament and Council for electromagnetic

compatibility and repealing

• IEC 61000-5-2: EMC-Installation and mitigation guidelines - Earthing and cabling

• IEC 61000-6-2: EMC-Generic standards - Immunity for industrial environments

• IEC 61000-4-16: EMC-Testing and measurement techniques - Test for immunity to conducted,

common mode disturbances in the frequency range 0 Hz to 150 kHz

• IEC 61000-4-13: Immunity to harmonics and inter-harmonics including mains signalling as a.c.

power port, low frequency immunity tests

• IEC 61000-4-8: Testing and measurement techniques – Power frequency magnetic field immunity

test.

• IEC 61000-6-4: EMC-Generic standards - Emission standard for industrial environments.

• IEC 61000-3: EMC-emissions standards – Emission of Harmonics, voltage changes, voltage

fluctuations and flicker

• IEC 62305-1 to 4: Protection against lightning


EMC CERN Guidelines I • CERN has a Mesh-Common Bonding Network (CBN), in many concrete

walls and floors (earthing bar) with CBN fixed terminals available

• All the metallic parts and equipment are interconnected and connected

to the common bonding network with low impedance bonding

Low impedance bonding


EMC CERN Guidelines II • Cable Selection criteria

Signal cables: bandwidth and carrier, impedance, signal losses,

length, etc

Power cables: according to IEC standards.

EMC: single or double shielding, twisted pairs with copper braid,

multi pair cables individually screened twisted pairs, etc.

Connectivity: mechanical protection, compatibility with cable types,

quality of signal (pins/contacts surface treatment), manipulations,

EMC protection level, etc

• Cable Installation

Maximize number of bonding loops minimizing their areas

Cable shields bonded on both ends

Not use CBN as return path


EMC CERN Guidelines III • Cable Trays Guides

Use of metallic cable trays with covers for all sensitive cables

Good electrical continuity all along the tray

Connection to the common bonding network at both ends and at

any other location as much as it is possible

Cable segregation on trays by function, immunity, sensitivity

• Boxes & cabinets for electric/electronic equipment

Use of metallic enclosures according to the vulnerability

Connect the cable trays to the enclosures

Connect the electrical reference to the common bonding network

Improve your equipment emission & immunity


Physics experiments • Usually, for the physics experiments, equipment are mobile and lot of cables are

used and stand on the floor.

• Try to find the same meshed reference for all equipment (equipotentiality)

• Try to put the cables along metallic earthed parts (to reduce ground loops areas)

• Connect cables shields and cable trays at both ends to the common bonding network


EMC AT CERN GUIDELINES

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