Grounding & Shielding

Ved Prakash SandlasDirector General

Amity Institute of Space Science & Technology, Noida

Principal Adviser, Cogent EMR Solutions Ltd, New Delhi (2006-2008)Distinguished Scientist and Chief Controller R & D, DRDO (1996-2005)

Director, Defence Electronics Applications Lab (DEAL), Dehradun (1986-1996)Group Director, Electronics, VSSC, Thiruvanathapuram (1984-1986)

Project/Mission Director, SLV-3, ISRO (1980-1984)

AISST, Noida, Feb 8, 2010

REASONS FOR GROUNDING

•Lightning Protection to Buildings, Structures and Equipment

•Shock and Safety hazard control in Equipment, Laboratories, Hospitals and Homes

•Faraday Shielding of Cables

•Common Ground Reference for Measurements

•Common Mode EMI Filters

•Electrostatic Hazard Control

Ground (Return) Line, Signal Return and Power Return

Analog Ground, Digital Ground and Power Ground

TRANSMISSION LINE

LOAD CURRENT

RETURN CURRENT

LOADSOURCE

GROUND (EARTH)

GROUND RODS

TRANSMISSION SYSTEM USING GROUND AS CURRENT RETURN PATH

1 2 3

4 5 6

SYSTEM GROUND

EARTH GROUND

SINGLE POINT OR STAR GROUNDING

MULTI POINT GROUNDING

12

3

GROUNDING BRAID

TO EARTH GROUND

CONDUCTING GROUND PLANE

GROUNDING LUGS

RS

VSRL

COAXIAL CABLE

OUTER BRAID GROUNDED

SOURCE CHASSIS LOAD CHASSIS

LOW FREQUENCY GROUND HIGH FREQUENCY GROUND

C

HYBRID GROUNDING

LOW FREQUENCY------------SINGLE POINTHIGH FREQUENCY------------MULTI POINT

SAFETY GROUND AND EMI ISOLATION

POWER DISTRIBUTION PANEL

COMPUTER (CPU) PERIPHERAL

EARTH WIRE

LOADPOWER SUPPLY

+_

LOADPOWER SUPPLY

+_

LOADPOWER SUPPLY

+_

COMMON (FLOATING) RETURN

CASSIS RETURN

GROUND (EARTH) RETURN

Barrier of Finite Thickness

SHIELDING MECHANISM FOR PLANE WAVE

Incident Wave

Attenuated Wave

EY

EY

EYEY

EYHZ

HZ

HZ

HZ

HZ

Absorption Loss, A = 3.34 t √fGµ dBWhere,

t = Shield thickness in mils

f = Frequency in MHz

G = Conductivity relative to Copper

(Copper = 1, Aluminum = 0.61, Brass = 0.26, Iron = 0.17, Stainless Steel = 0.02)

µ = Permeability relative to Vacuum (Copper = 1, Aluminum = 1, Brass = 1, Stainless Steel = 1 Iron = 1000, Mu Metal = 80,000, Perm Alloy = 80,000)

• 1 mil of Copper at 1 GHz shall give over 100 dB absorption loss

• 1/8 inch (~ 0.3 cm) Iron sheet at 50 Hz gives 50 dB absorption loss

Shielding Absorption Loss

300

100

30

10

3

1

dB

Frequency Hz10 100 1k 10k 100k 1M 10M 100M 1G

IRON

COPPER

1 mil

10 mils

1/8 inch

1/8 inch10 mils

1 mil

For Plane Wave, Reflection Loss:

R = 108 + 10 log dB

• Reflection loss for plane wave at low frequencies is the major shielding mechanism

• Also, high G and low µ is more effective (low surface impedance compared to 377 ohms)

•Also, at high frequencies, the skin depth decreases or surface resistivity increases (surface impedance increases), reducing reflection loss

•At VHF and UHF, absorption loss is more important

Gµf

Reflection Loss for Plane Wave

250

200

150

100

50

0

dB

Frequency Hz10 100 1k 10k 100k 1M 10M 100M 1G

IRON

COPPER

• For high impedance wave in near field (Electric Field)

R = 354 + 10 log dB

Where,

r = Distance from source to barrier in inches (<< / 2)

F = Frequency in Hz

• For low impedance wave in near field (Magnetic Field)

R = 20 log √µ / FG + 0.136 r √FG / µ + 0.354 dB

An interesting condition is achieved for magnetic field reflection loss for iron with 1 inch separation, where it approaches 0 dB at 30 kHz indicating matching of wave impedance and surface impedance

0.462 r

GF3µr2

Reflection Loss for Electric Fields

250

200

150

100

50

0

dB

Frequency Hz10 100 1k 10k 100k 1M 10M 100M 1G

IRON

COPPER

r = 30m 1m 1inch

r = 1 inch1m30mPlane Wave

Reflection Loss for Magnetic Fields

125

100

75

50

25

0

dB

Frequency Hz10 100 1k 10k 100k 1M 10M 100M 1G

IRON

COPPER

r = 30m 1m 1inch

r = 1 inch1m30m

Plane Wave

ELECTRIC FIELD COUPLING

•Also called capacitive coupling increases with increase in circuit impedance and is the primary contributor at high frequencies

•To reduce electric field coupling

•Isolate the culprit circuit

•Shortest possible, point to point wiring

•Faraday shielding

CC CS1

CC CS1

SHIELD

SHIELD NODE

LOW IMPEDANCE BOND

VS ZL

SINGLE CONDUCTOR SHIELDED

CC CS2

CC CS2

SHIELDS

SHIELD NODES

LOW IMPEDANCE BONDS

VS ZL

BOTH CONDUCTORS SHIELDED

CS1

CS1

TRANSFORMER SHIELDING AND GROUNDING

DIFFERENTIAL MODE NOISE COUPLING

COMMON MODE NOISE COUPLING

CASENOISE

NOISE

A A

A

A

B

B

B

B

C

C

C

C

D

DD

DP

P

S

S

RL

RL

TRANSFORMER SHIELDING AND GROUNDING

DIFFERENTIAL MODE NOISE SHIELDING

COMMON MODE NOISE SHIELDING

CASE

NOISE

NOISE

A A

A

A

B

B

B

B

C

C

C

C

D

DD

DP

P

S

S

RL

RL

SHIELD

NODE

NODE

BOTH COMMON & DIFFERENTIAL MODE NOISE SHIELDING

NOISE (d)

A

A

B

B

C

C

D

D

P S

RL

TWO SHIELDS

NODES

NOISE (c)

Ic

Ic

Ic

Ic IcIc

Ic

Id

Id

Id

MAGNETIC FIELD COUPLING

•Predominates for low frequencies and low impedance circuits. Also generates cross-talk and hum in audio and telecommunication circuits.

•To reduce magnetic field coupling

•Increase impedance in culprit circuit

•Avoid ground loop currents and reduce loop area

•Use dedicated return lines and twisted pairs

•Shields of permeability greater than one

•Sensitive circuits should be used in differential mode

•Use of isolation transformers and optical couplers

Vs

Zs

ZL

Loop Area = l x h

l

h

Large Loop Area and no Separate Ground Return

Vs

Zs

ZL

l

h

Reduce Loop Area by Reducing Height Above Ground Plane

Vs

Zs

ZL

l

h

Dedicated Return – No Ground or Ground One End Only

Vs

Zs

ZL

Twisted Wire Return – No Ground or Ground One End Only

Vs

Zs

ZL

Breaking of Ground Loop With Isolation Transformer

Source and/or Load Grounded to Body

SHIELDED CABLES AND GROUNDING

FARADAY SHIELDING OF COAX BY TRIAX

GROUND CURRENT LOOPS IN COAX

SHIELDED TWISTED PAIR – TWINAX

SINGLE POINT GROUNDING OF TWINAX

QUADRAX WITH EXTRA SHIELD

(a) 0 dB – Reference Case

(a) 0 dB – Reference Case (b) – 2 dB

Twisted – 6 T/ft

(a) 0 dB – Reference Case (b) – 2 dB

(c) – 5 dB

Twisted – 6 T/ft

(a) 0 dB – Reference Case (b) – 2 dB

(c) – 5 dB (d) – 49 dB

Twisted – 6 T/ft

(a) 0 dB – Reference Case

(b) – 2 dB

(c) – 5 dB

(d) – 49 dB

Twisted – 6 T/ft

(e) – 57 dB

(a) 0 dB – Reference Case

(b) – 2 dB

(c) – 5 dB

(d) – 49 dB

Twisted – 6 T/ft

(f) – 64 dB

(e) – 57 dB

(a) 0 dB – Reference Case

(b) – 2 dB

(c) – 5 dB

(d) – 49 dB

Twisted – 6 T/ft

(f) – 64 dB

(e) – 57 dB

(g) – 64 dB

(a) 0 dB – Reference Case

(b) – 2 dB

(c) – 5 dB

(d) – 49 dB

Twisted – 6 T/ft

(f) – 64 dB

(e) – 57 dB

(g) – 64 dB

(h) – 71 dB

(a) 0 dB – Reference Case

(b) – 2 dB

(c) – 5 dB

(d) – 49 dB

Twisted – 6 T/ft

(f) – 64 dB

(e) – 57 dB

(g) – 64 dB

(h) – 71 dB

(i) – 79 dB

Twisted – 18 T/ft

PSLV C 11

PSLV C 11

THIRD PIN

•Ground Pin, Green Wire or Earth Connection

•75 mA through body considered fatal

•Filters or Capacitors connected between equipment circuitry and case should not result in more than 5 mA (for 0.1 µf) through earth line

•When there are a large number of users sharing a common earth line in the same building, this safety wire can carry plenty of trash and interference created by ON/OFF transients, leakage currents and radiations pickups – use 2 pins and non-metallic body

•Obviously, one should not share the ground line with lightning earth strip or water plumbing

Thank You