Chp 2 Flashcards by Travis Howell

EMI duration classifications

Transient and continuous

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EMI descriptive classifications

Intentional / unintentional
natural 
low frequency / high frequency
magnetic / electric
external / internal

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Solution to External EMI

Immunity design

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RFI vs EMI

Radio frequency interference is a subset of EMI

In the freq range of bands: LF, MF, HF, and VHF (band 5-8), 30kHz to 300 MHz

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Solution to RFI on telephony

Shielded cabling and screening in concerned areas

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Types of EMI generated signals

Differential mode

Common mode

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Common Mode (CM)

Same signal on 2 conductors in circuit.

Locks up logic gates

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Differential Mode (DM)

Equal and opposite signals on conductors.

Corrupts signals

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All cables and shields can act like ______ if installed improperly.

Antennae

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The higher the transmission rate the _____ the effect of interference.

higher

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Loop antenna

Created by cabling and its ground reference

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Ground loop issues arise when

multiple ground reference planes are used (causing a CM voltage on cable sheathing)

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Ground Loop interference is solved by:

Use of pathways and cable trays to connect the multiple ground planes and to mitigate radiated signal effects

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When designing a cabling system for EMC, consider:

Availability of structural steel for grounding
bonding infrastructure for EFs, ERs, and TRs
ac grounding system design
use of surge protection
use of shielded cable
existing CM or DM levels
existing or possible sources of EMI

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EMI effects are mitigated best at low and high frequencies by

Low: cable twists absorb major part

High: cable shield absorbs most

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Main factors in unwanted signal coupling

1- effectiveness of shield/screen

2- termination implementation

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Grounding of cable shields shall

be made using a 360 degree connection avoiding breaches in continuity

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Typical EMI sources

Motors, transformers, lighting, copiers, equipment cords, terminals, power cables

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Avoid EMI by

maintaining physical separation

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EMI should be considered when designing the ______ system

pathway

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Effect of EMI on fiber optic cables

none

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Low voltage data lines should not be run in parallel to _________

high voltage power lines

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Terminate unused conductor at ________ or remove _______

both ends, altogether

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In critical circuits, use _______

optical fiber or well balanced twisted pairs

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Device used for mitigating EMI effects

isolation transformer

Isolation transformers are commonly used in ____ applications and perform the following functions to mitigate EMI:

LAN; - interrupt the CM ground loop - CM rejection and DM signal processing without distortions or other alterations on signal - impedance matching - balance-to-unbalance conversion

Iso transformers reject CM by

drains both CM branches to ground via primary coil center tap

Mode conversion is

when CM voltage is transferred from a transformers primary to secondary resulting in DM noise

EMC filtering components:

Ferrite toroids, beads, and sleeves

When cable sheath grounding is not efficient enough _____ are added to the horizontal cabling system

ferrite toroids, beads, and sleeves

DM type ferrites are installed ________

on each individual conductor

CM ferrites are installed ____________

around all conductors/cables

To mitigate emission problems, ferrites shall be installed on the _______ end

source cable

To mitigate susceptibility issues, the ferrite shall be installed at the ______ to the susceptible unit

input

(?*)Grounding cable sheaths/shields is not effective enough for signals with _______ frequencies

high

In concept and calculation a well-balanced pair is assumed. In practice, cables and connecting hardware exhibit a ______ in capacitance, resistance, and inductance between conductor and ground.

finite unbalance

Longitudinal conversion transfer loss in decibels is a measure of the conversion from ________ to _______ because of cabling _________.

CM, DM, unbalance

A joint-use tunnel is

telephone transmission facility place in joint-use with electrical power distribution

3 major components of joint-use tunnel problems

1- induced EMF from power lines 2- coupling between power and telecom systems 3- the susceptibility of the telecom system

In joint-use tunnels, the magnitude of the electrical power-line influence is determined by 2 factors:

1- magnitude of current that propagates over the power cable | 2- physical config of the line

Methods of minimizing power line influence in joint-use tunnel:

- using well balanced three phase systems | - utilizing filters to reduce harmonics

Coupling from mutual inductance and mutual capacitance is determined by:

- physical separation distance - length of exposure (length in which cables are ran parallel to one another) - impedance of the return ground loop - shielding effectiveness

______ reduce the susceptibility of cabling systems to external fields

Cable shields

The 2 main effects of induced EMI are due to

1- cable acting as an antenna (cable connection to system) | 2- conducted interference due to ground-loop effect

Worst offenders in terms of unwanted signals are:

high-frequency transients generated by starting and stopping machinery

Recommended longitudinal balance at low frequencies

60dB or greater

Recommended longitudinal balance at high frequencies

30dB to 40dB or greater

*****ADD CARDS FOR PHYSICAL SEP TABLE

Unshielded power lines or electrical equipment in proximity to open or non-metal pathways.

610mm | 24in

Unshielded power lines or electrical equipment in proximity to a grounded metal conduit pathway.

305mm | 12in

Power lines enclosed in a grounded metal conduit (or equivalent shielding) in proximity to a grounded metal conduit pathway.

152mm | 6in

Electric motors and transformers

1220mm | 48in

Additional separation precautions must be adhered to when systems are above ________.

5kVA

Separation requirements between metallic cabling and flourescent lamps

127mm | 5in

Separation requirements between metallic cabling and neon lamps

127mm | 5in

Separation requirements between metallic cabling and mercury-vapor lamps

127mm | 5in

Separation requirements between metallic cabling and high-intensity discharge (HID) lamps

127mm | 5in

Separation requirements between metallic cabling and arc welders

780mm | 31in

Separation requirements between metallic cabling and frequency induction heating

991mm | 39in

Where product supplier guarantees do not exist, analysis shall be performed identify ____________

EMI sources and possible disturbances | e.g., frequency range, harmonics, transients, bursts, transmitted power

Separation requirements specify the _______ distance cabling must be from EMI sources

minimum

Another term for mode conversion

Balance-to-unbalance conversion

Chp 2 Flashcards

(62 cards)