Facility Earth Mesh Presentation PB

Lessons Worth Remembering

Facility Earth Mesh (Grounding)Facility Earth Mesh (Grounding)

2nd Workshop on Power Converters

For Particle Accelerators

June 14 – 16, 2010

P l B ll

June 14 – 16, 2010 Facility Earth Mesh - 2nd Workshop on Power Converters for Particle Accelerators 1

Paul Bellomo

Acknowledgements

P ti f th t i l h i i fPortions of the material herein is from:

Keith ArmstrongEMC specialist contractor based in the United KingdomEMC specialist contractor based in the United Kingdom 44(0) 1785 660247 [email protected]://www cherryclough comhttp://www.cherryclough.com

Ian McMichaelEMC specialist contractor based in [email protected]://www.pqsolutions.com.au

Ponciano RodriguezGroup Leader at SLAC

i @ l t f d d


[email protected]

Topics

1. Reasons why I am here

2. FEM for safety

3. Reducing conducted and radiated noise


1. Reasons Why I am Here


Reasons Why I am Here

My experience in the past year

• A relatively new, operating synchrotron light source was experiencingi bl ff ti d t i iti d hi fnoise problems affecting data acquisition and machine performance

and availability. The problems were attributable to an inadequate FEM

• A recent review of a synchrotron light source upgrade plan revealed• A recent review of a synchrotron light source upgrade plan revealedmisconceptions about implementing the FEM grounding system

• Many people seem to think the personnel safety ground and “technicalMany people seem to think the personnel safety ground and technicalground” should be separate systems. This simply will not work and isunsafe

• A good FEM, in conjunction with other appropriate techniques, willprovide personnel safety and acceptable conducted and radiated noisesuppression


Some Standards for Equipment and Personnel Safety

USAUSA

• NFPA 70 – 2008, National Electrical Code

• IEEE 80 – 2000, Guide for Safety in AC Substation Grounding

• IEEE 81 – 1983, Guide for Measuring Earth Resistivity

IEEE 142 2007 R d d P ti F G di f I d t i l• IEEE 142 - 2007, Recommended Practice For Grounding of Industrialand Commercial Power Systems (green book)

• IEEE 1100 – 2005, Recommended Practices for Powering andgGrounding Electronic Equipment (emerald book)


2. FEM for Safety


The Shock Hazard

30 000 f t l l t i l h k id t h i th U S• 30,000 non-fatal electrical shock accidents each year in the U.S. -1,000 fatalities, 2/3 due to unsafe acts,1/3 due to unsafe equipment

• Shock hazards are found in AC distribution equipment panel boards• Shock hazards are found in AC distribution equipment, panel-boards,power supplies, magnets, modulators, pump motors and many othertypes of electromechanical equipment

• Body from hand to hand or hand to foot at 50V presents a highresistance. Voltages > 50V penetrate skin, body resistance is muchlower and unpredictable. Dangerous current flow is possiblep g p

• Shock hazard levels10 to 17mA let-go threshold30mA chest paralysis and suffocation75 to100mA heart fibrillation – need defibrillator in < 5 minutes. Knowwhere the Artificial External Defibrillators are located


A Typical AC Distribution System

Substation 4.16kV Bus13.8 kV Bus Substation

4.16kV

4.16kV Bus13.8 kV Bus

• MV Systems Hi Z to ground

480V4.16kV

MV Systems Hi Z to ground• LV solidly grounded• Grounded at every level

F lt t d

208V:120V480V

Li t d f lt

• Faults to ground• Unbalanced lines• Lightning

ControlPS

480VLine to ground fault

g g

to powersupplies

PSPS

208V:120V


Load Load Load

Step and Touch Potentials

Electric currents flow in the earthduring system fault or unbalancedcircuit conditions These currentscircuit conditions. These currentscause differences in the potentialsthe earth’s surface

Step potential can be experienced by a person simply by having their feet a“step” apart, even without contacting any grounded object

Touch potential is the difference between the ground potential where aperson is standing and the potential at the point where a hand (or otherbody part) is in contact with another structurebody part) is in contact with another structure.

Step and touch hazards are reduced by FEM (grounding) systems that limitthe potentials to 50V or less


General Safety Concepts

• Keep Step and Touch Potentials to < 50V

• Combine all safety and technical ground systems into one, common FEM


Separate Grounds are Unsafe and Do Not Provide EMC

‘Clean’ earths using single-point bonding

‘Clean’ grounds using single-point bonding

SafetyEMC

Lightning Lightning

‘Power’ earths using single-point bonding

Power grounds using single-point bonding

Other floorsOther floors

protection system (LPS)

g gprotection

system (LPS)

Ground floorGround floorMains distribution

Mains distribution

Item of equipment

1 3 N2 Bonding conductorBonding

conductor

LPS earth (mass) l t d

LPS earth (mass) l t d

SoilSoilHVHV Earth (mass) Earth (mass)


electrodeselectrodeselectrode( )

electrode

Tying System Grounds Provides Safety but SP Grounds - Not EMC

Safety ‘Clean’ earths using single-point bonding

Clean grounds using single-point bonding

Safety

EMC

Lightning protectionLightning protection

‘Power’ earths using single-point bonding Power grounds using single-point bonding

Other floorsOther floors



Mains distribution

Mains distribution

Ground floorGround floorGround floorGround floor

Item of equipment

1 3 N2 Bonding conductorBonding

conductor

Earth (mass) electrodesEarth (mass) electrodesSoilSoilHVHV Earth (mass) l d

Earth (mass) LPS bonds to MESH-CBNLPS bonds to MESH


( )Earth (mass) electrodeselectrode( )

electrodeLPS bonds to MESH CBN LPS bonds to MESH

FEM System Provides Good Safety and EMC

Other floorsOther floorsSafetyEMC

Lightning t ti

Lightning Meshed common bonding network,

MESH-CBN, for both ‘power’ and ‘clean’ earthsMeshed common bonding network,

MESH-CBN for both power and clean’grounds protection system (LPS)


Ground floorGround floorMains distribution

Mains distribution

Item of equipment

MESH CBN, for both power and clean earths(also bonds services and ‘natural’ metalwork)

MESH CBN, for both power and clean grounds(also bonds services and all metalwork)

Bonding conductors

Bonding conductors

1 3 N2

Earth (mass) electrodesEarth (mass) electrodesSoilSoilHVHV

Earth (mass) l t d

Earth (mass) LPS bonds to MESH-CBN LPS bonds to MESH-CBN


( )Earth (mass) electrodeselectrodeelectrode (additional bonds every 15-20m height in tall buildings)

LPS bonds to MESHCBN (additional bonds every 15-20m height in tall buildings)

Some FEM Details

S b t ti lSubstation, power supply, or other building type

This is a rudimentary FEM, provides safety but little EMC

Ground cable to Ring

2/0 to 500kcmil 8’ to 10’

Ground cable to Master Substation


2/0 to 500kcmil stranded copper cable buried 18” to 24” deep

8 to 10

SLAC Rg=1s to 10s of ohms

Tie All Facilities FEMs Together


Electrical Rack and Cabinet Bonding

T l bi t IT bi t

‘Frame bond’ conductors, 28 sq. mm, short and direct to underfloor ‘bonding mat’ or SRPP (area of MESH CBN)Bond cabinets to FEM

Telco cabinets IT cabinets

UPSComputer

flooring pedestal

Computer floor

pedestalpedestalpedestal


Typical mesh size: 600mm square, connected to the BRC of the app ropriate EM ZoneTypical mesh size: 600mm square

3. Reducing Conducted and Radiated Noise


Five Types of Electromagnetic Interference

1. Conducted Emissions - the EMI emitted into lines and connectionsby an electronic device

2 Conducted Susceptibility the EMI present on lines and connections2. Conducted Susceptibility - the EMI present on lines and connectionsthat affect a connected electronic device

3. Radiated Emissions - the EMI radiated by an electronic devicey

4. Radiated Susceptibility - the affect of radiated EMI on an electronicdevice

5. Electromagnetic Pulse – radiated EMI by lightning or atomic blast


Some EMC Standards

USA

Federal Communications Commission (FCC) Code of Federal RegulationsCFR, Part 15, Sub-Part J, for Class A and B devices and equipment, , , q p

The FCC Class A requirements pertain to industrial equipment, while theClass B requirements are applicable to consumer equipment.

MIL-STD-461E and 462E Emissions and Susceptibility Standard forDefense Electronics. This standard sets the emissions and susceptibility(immunity) noise limits and test levels

International

There are many different standards used throughout the world. Therequirements and details vary but the concepts and intended end resultrequirements and details vary, but the concepts and intended end resultare the same as in the USA


Conducted Emissions and Susceptability

EMI conducted onto lines by electrical, electronic and power conversionequipment

Typically 10 kHz to 30 MHz, covered by standards, but the noise frequency can be as low as the AC line frequency

Measured in V or dB V ( Reference: 1 V 0 dB )Measured in V or dB - V ( Reference: 1 V = 0 dB )

10measured VdB V 20* log

10dB V 20 log1 V


EMI Emissions – Conducted levels

100100

8080

6060FCC Class A - IndustrialV

4040

FCC Class A Industrial

dB -V

FCC Class B - Consumer

2020

FCC Class B Consumer

0.01 0.1 1 1000.01 0.1 1 10

0


0 0 0 00 0 0 0

Frequency in MHz

Conducted Emissions Test Equipment

S t l ith Li I d St bili ti N t k (LISN )Spectrum analyzers with Line Impedance Stabilization Networks (LISNs)• Filter and divert external AC line intrinsic noise from the EMI

measurementsI l t d d l th AC li hi h lt d t li t i t• Isolate and decouple the AC line high voltage and prevent line transientsfrom damaging spectrum analyzers and other sensitive test equipment

• Present a known, fixed impedance at RF frequencies to the power supplyd i t tundergoing test


Differential Mode Noise

Conducted or radiated noise t t lionto system lines

V1= - V2

Magnitudes are equal

Phase difference is 180O

VDIFF = V1 – V2 = unwantedsignal

IDIFF = VDIFF / Rload flows into loadand FEM


Differential Mode Electromagnetic Compatibility

• Current flow in opposite directions so that the magnetic field iscontained within the spirals

• The tighter the cable twist the greater the containment and noise• The tighter the cable twist the greater the containment and noise attenuation into the FEM

• Shielding the pair (and tying the shield to ground in one or more places) will also increase noise attenuation

• To minimize unwanted noise in loads filter across the load (line-line)


Common Mode Noise

Produced as a result of simultaneous high frequency voltages on (+) and ( ) linesvoltages on (+) and (-) lines capacitively coupled to ground

V1 = V2 = VCOM1 2 COM

Magnitudes are equal

Phase difference is 0OPhase difference is 0

V3 = 0

I V *2 fCIc = Vcom*2fC


Common Mode Electromagnetic Compatibility

• Current flows are the same magnitude and in the same direction so that the spirals have no effect on containing the magnetic fields

• The pair must be shielded and the shield tied to ground in one or more places for noise attenuation

• To minimize common mode voltages at the load, filter from each line to ground as shown above.


Conducted Line Noise Filters

Delta inputp

Wye input

C fi ti C L Pi T• Configurations C, L, Pi, T

• Attenuation 20 to 70dB

• Filters both differential and• Filters both differential and common mode noise


Reducing Conducted Noise on Other Systems / EquipmentMain AC Bus

PS PS PS I & CI & C I & C

• Separate noisy power supplies from sensitive I & C loads by Faraday -shielded transformers to attenuate common mode noise

• Place PS and I & C loads on dedicated busses separted by at least


p yone transformer

Other Conducted Noise Filters

Plain Transformer Electrostatic (Faraday) Plain Transformer Shielded Transformer

ConverterIn Output

IDIFF ICOMCommon Mode Choke

IDIFFICOM

Differential mode currents flow in opposite directions. Magnetic fields cancel, choke presents low impedance, low attenuation to

Common mode currents flow in same direction. Magnetic fields add, choke presents high impedance, high attenuation to


noise noise

Radiated Emissions

EMI di t d f bl t f th t• EMI radiated from cables, transformers, other components

• Typically 30 MHz to > 1GHz. Cables and other equipment are effectiveradiators of frequencies above 30 MHz

• Measured in V / m or dB - V / m ( Reference: 1 V / m = 0 dB )

• Measured 3 m (Class B residential) or 30 m (Class A industrial) from theitti i t TV l t d ithi 3 f t i th hemitting equipment. TVs located within 3 m of computers in the home

and within 30 m in the industrial setting. Limits 100 to 200 V / m are1/10 of TV reception signal

• Industrial FCC Class A limits of 200 V / m are higher (less severe)because it is assumed that there will be an intervening wall betweenculprit and victim that will provide some shielding

• Test equipment are spectrum analyzers, rotating tables, conical and/orlog periodic antennas and anechoic chambers designed to minimizereflections and absorb external EMI


Radiated Emissions Test Setup


Anechoic Chamber and Measuring Antenna


Radiated Emissions - Faraday’s Induced Voltage Law

d φ d BV E dl A the magnitude and rate of change of flux density with time

the area of the loop cut by flux

V E dl Adt dt

dBVdt

V A

the area of the loop cut by flux

It can be shown for a single loop in air that the noise voltage is

V A

V 2* π* f * μ

* H * AnV 2 π f μ

Wherenoise voltage in volts

0 H A

V noise voltage in volts frequency in Hz permeability of free space ( )magnetic field strength in A/m

n

7 20

Vfμ 4* π* 10 N / ampH

magnetic field strength in A/m loop area in m

Minimize loop areas - run supply and return bus

2

HA

or cable conductors together


Minimize loop areas - run supply and return bus or cable conductors togetherand twist cables whenever possible

FEM Size is Based on Application

FEM diagonals, D (m) < 50 m/f (MHz) of the highest operating frequency providef t d d t EMC b fit D ( ) 150 /f (MH ) id f t b t EMC

Example of computer roomExample of computer room

o other floors

other floors

safety and adequate EMC benefit. D (m) > 150 m/f (MHz) provides safety, but no EMCbenefit

To otTo oth

Example of instrumentation roomExample of instrumentation roomExample of power distribution roomExample of power distribution room


Creating the 3D FEM – Bond All Equipment to the FEM

MESH-CBN vertical elements (e.g. steel girders, re-bars)

MESH-CBN vertical elements (e.g. steel girders, re-bars)

MESH-CBN bonded to each floor's BRC at several placesMESH-CBN bonded to each

floor's BRC at several places

Cables routed close to MESH-CBN elements

l ti

Cables routed close to MESH-CBN elements

along entire route

along entire route

Cables and Cables and metallic services

RF-bonded at every BRC they

cross(Earthing bars

metallic services RF-bonded at

every BRC they cross

(E thi b(Earthing bars, plates, filters, SPDs, etc., not

shown)

(Earthing bars, plates, filters, SPDs, etc., not

shown)


MESH-CBN’s horizontal elements (e.g. re-bars)

MESH-CBN’s horizontal elements (e.g. re-bars)

Example of a meshed RF Reference (e.g. SRPP, bonding mat)

Example of a meshed RF Reference (e.g. SRPP, bonding mat)

Creating the FEM – Cable Bond Network

B di d tMetal window and doorM t l i d d d Bonding conductors and BRCs

Bonding conductors and BRCs

Structural steelworkincluding reinforcementStructural steelwork

including reinforcement

Metal window and door frames, cladding,

roofing, etc.

Metal window and door frames, cladding,

roofing, etc.

Plumbing and pipework

Plumbing and pipework

Bonds across non- metallic pipe sections

Bonds across non- metallic pipe sections

Cable ducts, trays, conduits, etc.

Cable ducts, trays, conduits, etc.

p ppipe sectionsAir ducts, vents,

flues, chimneys, etc.Air ducts, vents,

flues, chimneys, etc.

Gratings, ladders, Gratings, ladders,Short bonding conductors used where direct metal to metal

Short bonding conductors used h di t t l t t l


g , ,walkways, fences, etc.

Gratings, ladders, walkways, fences, etc.where direct metal-to-metal

fixings or welds are not practicalwhere direct metal-to-metal

fixings or welds are not practical

Creating the FEM

Paint removed to expose plated t l RF b d d 3/f

Paint removed to expose plated /fmetal, RF-bonded every 3/fmax

metres along the joint to the cable tray or duct

metal, RF-bonded every 3/fmaxmetres along the joint to the

cable tray or duct

( f in MHz)( f in MHz)


f

Creating the FEM

U-bracket, bonded metal-to-metal at least every 3/fmax

metres along the joint

U-bracket, bonded metal-to-metal at least every 3/fmax

metres along the jointBase of duct or tray bent down and fixed to cabinet

3/fmax metres along the joint

Base of duct or tray bent down and fixed to cabinet

3/fmax metres along the joint

g j

(Continuous seam-weld or conductive gasket is best)

metres along the joint

(Continuous seam-weld or conductive gasket is best)

Double strap

Double strap

22 33

Single strapor wire bondSingle strapor wire bond

1133

44 f


44 ( f in MHz)( f in MHz)

Radiated Emissions

Any component or cable > 1/20 wavelength (l) will be an efficientradiating or receiving antenna

Wavelength Vs Cable Lengths

Frequency 1/4 1/20

10 kHz 30 km 7500 m 1500 m = 59,000 in

100 kHz 3 km 750 m 150 m = 5,900 in

1 MHz 300 m 75 m 15 m = 590 in

10 MHz 30 m 7.5 m 1.5 m = 59 in

30 MHz 10 m 2.5 m 50 cm = 20 in

100 MHz 3 m 75 cm 15 cm = 6 in


1 GHz 30 cm 7.5 cm 1.5 cm = 0.6 in

Low Impedance Returns Are Mandatory

Return conductors

• Above a few kHz resistance is not a factor, capactive and inductivereactances (impedance) dominatereactances (impedance) dominate

• Inductive reactance (impedance, 2 f L) is proportional to inductanceand frequency. Inductance, L, is proportional to length, and inverselyproportional to thickness and widthproportional to thickness and width.

• Want to reduce path length

• Thickness can only be increased so much because of skin effectThickness can only be increased so much because of skin effect

• Width can be increased indefinitely. However, wide conductors arestiff and not easily installed. Get around this by braiding the

d tconductors


Low Impedance Returns Are Mandatory

SignalSource Load

I t d d

PowerSource

arc

IntendedReturn

NoiseArc NoiseArc

i i

g g


Ig = Iarc *Zi / ( Zi + Zg) Zi >> Zg ; Ig Iarc Zi << Zg ; Ig 0

Conductor Comparisons for Low Impedance

Long wires are poor for RF, but help control LF (e.g. 50Hz)

Long wires are poor for RF, but help control LF (e.g. 50Hz) 44

Shorter wire length is better for RF

Shorter wire length is better for RFbette obetter for RF

Short, wide braid strap is better still for RF

Short, wide braid strap is b tt till f RF 22

33

better still for RFbetter still for RF

Short wide metal plates with Short wide metal plates with

22

pmultiple fixings are the best

Short wide metal plates with multiple fixings are the best 11


But no bonding conductor is ever as effective at RF as multiple direct metal-to-metal bonds, especially seam-welding

But no bonding conductor is ever as effective at RF as multiple direct metal-to-metal bonds, especially seam-welding

Examples of RF Bonding Techniques for Cable Tray and Duct Joints

Wires only provide bonds that are

effective up to 1kHz

44

effective up to 1kHz

Short, wide braid straps could bond up to MHz 33 p

Use as many as practical, spread along the joint and

at both ends

U-brackets with RF-bonds every 3/f metres along the join and at both ends (f in MH ) ff ti t f

22

But continuously seam-welded or conductively-gasketed joints are best

Covers and lids are good for EMC up to f if they have RF bonds 30/f metres

MHz) are effective up to f

11


Covers and lids are good for EMC up to f if they have RF-bonds 30/f metres along their whole length, and every 3/f metres at joints and both ends

( f in MHz)

Connecting Signal Shields to the FEM

Source Receiver Source Receiver

Low Frequency High FrequencyFEM FEM

• Low frequency single-point ground adequate when L 1/20 of f H

• High frequency multiple path, low impedance


AC Power Cables

Loops are undesirable, but unavoidable. Smaller loops are better. Makethe loops as small as possible. Bundle 3 phase cables and enclose indedicated gro nded metal cond its If possible t ist the cablesdedicated grounded, metal conduits. If possible, twist the cables.

Ensure that grounding cables closely follow the path of the input ACphase conductors to reduce magnetic loops Incorporate a safety groundphase conductors to reduce magnetic loops. Incorporate a safety groundcable in parallel with, or strapped to its AC power conductors

Connect the grounding conductors in multi-conductor AC power cables toConnect the grounding conductors in multi conductor AC power cables tothe FEM and the other end to the grounding bus in each switchgear,panel-board, or other premises or utilization equipment.


Radiated Noise Reduction – Run DC Cables As Pairs

Opposing currents in adjacent cables


in adjacent cables

Minimize Cable Loop Area

NNLL

Bad EMC practiceBad EMC practiceBad EMC practice0V0V

The send and return conductors The send and return conductors

NNLL

should be in close proximity over their entire route

— ideally twisted together— for every kind of power

The send and return conductors should be in close proximity

over their entire route — ideally twisted together— for every kind of powerfor every kind of power or signal interconnection

— for every kind of power or signal interconnection

__++


0V

Signal and Power Cables

Return Return Signal 1 Signal 2 Return Signal 3 Signal 4 Return …etc. is the minimum configuration that should be used for flat cable

Select EMC cable quality based upon the application.

A few extra ‘grounding’ conductors spreadconductors spread

throughout a wire bundle improves its EM

performance

Return Return Signal 1 Return Signal 2 Return Signal 3 Return…etc. gives the best EM performance that ribbon cable can achieve (but not as good as using twisted send/return pairs)

A large number of extra ‘grounding’ conductors spread throughout a wire bundle improves its EM performance even more (but still not as good as


even more (but still not as good as using twisted send/return pairs)

Radiated Noise Reduction Techniques

( f f )Use shielded cables and enclosures (if necessary for interior controls)

http://www.lindgrenrf.com/


Creating Shielded Enclosures

Conductively-plated fabric over foam

Conductively-plated f b i ffabric over foamfabric over foam

Volume-conductive elastomers

Volume-conductive elastomers

These examples are from LairdTh l f L i dThese examples are from Laird Technologies, www. lairdtech.com

These examples are from Laird Technologies, www. lairdtech.com

Knitted wire over foamKnitted wire over foam

Hollow knitted wireHollow knitted wire


Creating Shielded Enclosures

Examples of ‘spring Examples of ‘springp p gfingers’ (‘fingerstock’)

from Laird Technologies, www.lairdtech.com

Examples of spring fingers’ (‘fingerstock’)

from Laird Technologies, www.lairdtech.com


Other Techniques – Concluding Remark

• Avoid air gaps in transformer/inductor cores. Use toroid windings forair core inductors

Wh ti l fib ti bl t l di it l• Whenever practical, use fiber optic cables to carry analog, digital orcontrol signals, instead of cables or other conductors. Fiber opticcables do not behave as accidental antennas, they provide galvanicisolation that completely ignores any electromagnetic disturbances atisolation that completely ignores any electromagnetic disturbances, atany frequency, and they can handle high data rates.


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