Fluke 43B Application Note 2403183

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Application Note

Electrical noise is the result of

more or less random electrical

signals getting coupled into cir-

cuits where they are unwanted,

i.e., where they disrupt informa-

tion-carrying signals. Noise

occurs on both power and signal

circuits, but generally speaking, it

becomes a problem when it gets

on signal circuits. Signal and data

circuits are particularly vulnerable

to noise because they operate at

fast speeds and with low voltage

levels. The lower the signal volt-

age, the less the amplitude of the

noise voltage that can be toler-

ated. The signal-to-noise ratio

describes how much noise a cir-

cuit can tolerate before the valid

information, the signal, becomes

corrupted.

Noise is one of the more

mysterious subjects in power

quality, especially since it must

be considered with its equally

mysterious twin, grounding. To

lessen the mystery, there are two

key concepts to understand:

•The first is that electrical

effects do not require direct

connection (such as through

copper conductors) to occur.

For an electrician who’s been

trained to size, install and test

wiring, this may not be intu-

itive. Yet think of lightning, or

of the primary and secondary

of an isolation transformer, or

of the antenna to your radio:

there’s no direct, hard-wired

connection, but somehow

complete electrical circuits are

still happening. The same

electrical rules-of-behavior are

in operation for noise coupling,

as will be explained below.

•The second concept is that we

can no longer stay in the

realm of 60 Hz. One of the

benefits of 60 Hz is that it’s a

low enough frequency that

power circuits can be treated

(almost) like dc circuits; in

other words, basic Ohm’s Law

will get you most places you

need to go. But when it comes

From the Fluke Digital Library @ www.fluke.com/library

Electrical noise

and transients

to noise, we need to keep in

mind that signal circuits occur

at high frequencies, that noise

is typically a broad spectrum of

frequencies, and that we need

to consider the frequency-

dependent behavior of

potential sources of noise.

Coupling mechanisms

There are four basic mechanisms

of noise coupling. It pays to

understand them and how they

differ one from the other because

a lot of the troubleshooter’s job

will be to identify which coupling

effect is dominant in a particular

situation.

1. Capacitive coupling

This is often referred to as

electrostatic noise and is a volt-

age-based effect. Lightning

discharge is just an extreme

example. Any conductors sepa-

rated by an insulating material

(including air) constitute a capac-

itor—in other words, capacitance

is an inseparable part of any cir-

cuit. The potential for capacitive

coupling increases as frequency

increases (capacitive reactance,

which can be thought of as the

resistance to capacitive coupling,

decreases with frequency, as

can be seen in the formula:

XC= 1/2πfC).

2. Inductive coupling

This is magnetic-coupled noise

and is a current-based effect.

Every conductor with current

flowing through it has an associ-

ated magnetic field. A changing

current can induce current in

another circuit, even if that circuit

is a single loop; in other words,

20 - 30 V

logic signal

3 - 5 V

logic signal

Noise

Noise

Figure 1. Lower voltage, faster signals increase sensitivity to noise.

2 Fluke Corporation Electrical noise and transients

the source circuit acts as a trans-

former primary with the victim

circuit being the secondary. The

inductive coupling effect

increases with the following fac-

tors: (1) larger current flow, (2)

faster rate of change of current,

(3) proximity of the two conduc-

tors (primary and secondary) and

(4) the more the adjacent con-

ductor resembles a coil (round

diameter as opposed to flat, or

coiled as opposed to straight).

Here are some examples of

how inductive coupling can cause

noise in power circuits:

•A transient surge, especially if

it occurs on a high-energy cir-

cuit, causes a very fast change

in current which can couple

into an adjacent conductor.

Lightning surges are a worst

case, but common switching

transients or arcing can do the

same thing.

•If feeder cables are positioned

such that there is a net mag-

netic field, then currents can

be induced into ground cables

that share the raceway.

•It is well known that signal

wires and power conductors

should not be laid parallel to

each other in the same race-

way, which would maximize

their inductive coupling, but

instead be separated and

crossed at right angles when

necessary. Input and output

cables should also be isolated

from each other in the same

manner.

Magnetic fields are isolated by

effective shielding. The material

used must be capable of conduct-

ing magnetic fields (ferrous

material as opposed to copper).

The reason that a dedicated cir-

cuit (hot, neutral, ground) should

be run in its own metal conduit

when possible is that is in effect

magnetically shielded to mini-

mize inductive coupling effects.

Both inductive and capacitive

coupling are referred to as near

field effects, since they dominate

at short distances and distance

decreases their coupling effects.

This helps explain one of the

mysteries of noise—how slight

physical repositioning of wiring

can have such major effects on

coupled noise.

3. Conducted noise

While all coupled noise ends up

as conducted noise, this term is

generally used to refer to noise

coupled by a direct, galvanic

(metallic) connection. Included in

this category are circuits that

have shared conductors (such as

shared neutrals or grounds).

Conducted noise could be high

frequency, but may also be 60 Hz.

Common examples of connec-

tions that put objectionable noise

currents directly onto the ground:

•Sub-panels with extra N-G

bonds

•Receptacles miswired with

N and G switched

•Equipment with internal solid

state protective devices that

have shorted from line or neu-

tral to ground, or that have not

failed but have normal leakage

current. This leakage current is

limited by UL to 3.5 mA for

plug-connected equipment, but

there is no limit for perma-

nently wired equipment with

potentially much higher leak-

age currents. (Leakage currents

are easy to identify because

they will disappear when the

device is turned off).

•Another common example is

the so-called isolated ground

rod. When it is at a different

earth potential than the source

grounding electrode, a ground

loop current occurs. This is still

conducted noise, even though

the direct connection is

through the earth.

•Datacom connections that

provide a metallic path from

one terminal to another can

also conduct noise. In the case

of single-ended, unbalanced

connections (RS-232), the con-

nection to terminal ground is

made at each end of the cable.

This offers a path for ground

currents if the equipment at

each end is referenced to a

different power source with a

different ground.

4. RFI (Radio Frequency

Interference)

RFI ranges from 10 kHz to the

10 s of MHz (and higher). At

these frequencies, lengths of wire

start acting like transmitting and

receiving antennas. The culprit

circuit acts as a transmitter and

the victim circuit is acting as a

receiving antenna. RFI, like the

other coupling mechanisms, is a

fact of life, but it can be con-

trolled (not without some thought

and effort, however).

Ø - Ø

Ø - G N - G

Noise Coupling

Ground

ØA

ØB

N

ØC

Figure 2. Noise coupling. Ground noise measured as ø-G or N-G noise.

3 Fluke Corporation Electrical noise and transients

RFI noise reduction employs a

number of strategies:

•Fiber optic cable, of course, is

immune to electrical noise.

•Shielded cabling (such as coax

cables) attempts to break the

coupling between the noise

and signal.

•Balanced circuits (such as

twisted pair) don’t break the

coupling, but instead take

advantage of the fact that the

RFI will be coupled into both

conductors (signal and return).

This noise (called Common

Mode noise) is then subtracted,

while the signal is retained. In

effect, the balanced circuit cre-

ates a high impedance for the

coupled noise.

•Another example of the high-

impedance-to-noise approach

is the use of RF chokes.

Whether used with data or

power cables, RF chokes can

offer effective high-frequency

impedance (XLincreases with

frequency).

•A low-impedance path can be

used to shunt away the noise.

This is the principle behind

filtering and the use of decou-

pling caps (low impedance to

high frequency, but open at

power line frequencies). But a

sometimes-overlooked, yet

critical, aspect is that the

ground path and plane must

be capable of handling

high-frequency currents.

High-frequency grounding

techniques are used to accom-

plish this. The SRG (Signal

Reference Grid), first devel-

oped for raised floor computer

room installations, is an effec-

tive solution. It is essentially

an equipotential ground plane

at high- frequency. (For further

information on high-frequency

grounding, see the references

listed on the back page.)

Signal grounding

To understand the importance

of “clean” signal grounds, let’s

discuss the distinction between

Differential Mode (DM) vs.

Common Mode (CM) signals.

Imagine a basic two-wire circuit:

supply and return. Any current

that circulates or any voltage

read across a load between the

two wires is called DM (the terms

normal mode, transverse mode

and signal mode are also used).

The DM signal is typically the

desired signal (just like 120V at a

receptacle). Imagine a third

conductor, typically a grounding

conductor. Any current that flows

now through the two original

conductors and returns on this

third conductor is common to

both of the original conductors.

The CM current is the noise that

the genuine signal has to over-

come. CM is all that extra traffic

on the highway. It could have

gotten there through any of the

coupling mechanisms, such as

magnetic field coupling at power

line frequency or RFI at higher

frequencies. The point is to con-

trol or minimize these ground or

CM currents, to make life easier

for the DM currents.

Measurement

CM currents can be measured

with current clamps using the

zero-sequence technique. The

clamp circles the signal pair (or,

in a three-phase circuit, all

three-phase conductors and the

neutral, if any). If signal and

return current are equal, their

equal and opposite magnetic

fields cancel. Any current read

must be common mode; in other

words, any current read is cur-

rent that is not returning on the

signal wires, but via a ground

path. This technique applies to

signal as well as power conduc-

tors. For fundamental currents, a

ClampMeter or DMM + clamp

would suffice, but for higher

frequencies, a high bandwidth

instrument like the Fluke 43

Power Quality Analyzer or

ScopeMeter should be used with

a clamp accessory.

A Matter of Life and Death

Sometimes PQ troubleshooting is

a matter of life and death.

Dave was the on-site field

engineer at the hospital. One

day he got a call from a very

concerned nurse in the ER. One

of their patients had died. But as

upsetting as that was, it wasn’t

the main source of concern.

What was really unusual was

that this particular corpse had a

heartbeat.

Dave soon arrived at the

scene. A quick glance told him

that the dead had not come back

to life. The problem lay else-

where. The nurses pointed out

what they had seen, a signal on

the EKG indicating a heartbeat.

But there was something

unusual about this signal (above

and beyond the fact that it

seemed to be coming from a

dead body). He noticed that the

signal was a 60 Hz sine wave

(slightly flat-topped). A further

look at the signal wires told him

that they had been laid parallel

to the power cord. The coupling

between signal and power wires

caused the 60 Hz “Heartbeat” on

the EKG machine. The moral of

the story is to always isolate the

signal and power conductors—

before it becomes a matter of life

and death.

4 Fluke Corporation Electrical noise and transients

Transients

Transients should be distin-

guished from surges. Surges are a

special case of high-energy tran-

sient which result from lightning

strikes. Voltage transients are

lower energy events, typically

caused by equipment switching.

They are harmful in a number

of ways:

•They deteriorate solid state

components. Sometimes a sin-

gle high energy transient will

puncture a solid state junction,

sometimes repetitive low

energy transients will accom-

plish the same thing. For

example, transients which

exceed the PIV (peak inverse

voltage) rating of diodes are a

common cause of diode failure.

•Their high-frequency compo-

nent (fast rise times) cause

them to be capacitively cou-

pled into adjoining conductors.

If those conductors are carry-

ing digital logic, that logic will

get trashed. Transients also

couple across transformer

windings unless special

shielding is provided.

Fortunately this same high

frequency component causes

transients to be relatively

localized, since they are

damped (attenuated) by the

impedance of the conductors

(inductive reactance increases

with frequency).

•Utility capacitor switching

transients are an example of

a commonly-occurring high-

energy transient (still by no

means in the class of light-

ning) that can affect loads at

all levels of the distribution

system. They are a well

known cause of nuisance

tripping of ASDs: they have

enough energy to drive a

transient current into the dc

link of the drive and cause an

overvoltage trip.

Transients can be categorized

by waveform. The first category

is “impulsive” transients, com-

monly called “spikes,” because a

high-frequency spike protrudes

from the waveform. The cap

switching transient, on the other

hand, is an “oscillatory” transient

because a ringing waveform rides

on and distorts the normal wave-

form. It is lower frequency, but

higher energy.

Causes

Transients are unavoidable. They

are created by the fast switching

of relatively high currents. For

example, an inductive load like a

motor will create a kickback

spike when it is turned off. In

fact, removing a Wiggy (a sole-

noid voltage tester) from a

high-energy circuit can create a

spike of thousands of volts! A

capacitor, on the other hand, cre-

ates a momentary short circuit

when it’s turned on. After this

sudden collapse of the applied

voltage, the voltage rebounds and

an oscillating wave occurs. Not

all transients are the same, but as

a general statement, load switch-

ing causes transients.

In offices, the laser copier/

printer is a well-recognized “bad

guy” on the office branch circuit.

It requires an internal heater to

kick in whenever it is used and

every 30 seconds or so when it is

not used. This constant switching

has two effects: the current surge

or inrush can cause repetitive

voltage sags; the rapid changes

in current also generate tran-

sients that can affect other loads

on the same branch.

Measurement and recording

Transients can be captured by

DSOs (Digital Storage Oscillo-

scopes). The Fluke 43 PQ

Analyzer, which includes DSO

functions, has the ability to cap-

ture, store and subsequently

display up to 40 transient wave-

forms. Events are tagged with

time and date stamps (real time

stamps). The VR101S Voltage

Event Recorder will also capture

transients at the receptacle. Peak

voltage and real time stamps are

provided.

Figure 3. Fluke 43B can capture and save up

to 40 transients.

Cursor moves to

display peak

Min/Max values.

Real-time stamp.

Date:hr:min:sec

5 Fluke Corporation Electrical noise and transients

Voltage susceptibility profile

The new ITIC profile (Information

Technology Industry Council) is

based on extensive research and

updates the CBEMA curve. The

CBEMA curve (Computer Business

Equipment Manufacturers

Association, now ITIC) was the

original voltage susceptibility

profile for manufacturers of com-

puters and other sensitive

equipment. Similar curves are

Transient voltage surge

suppressors (TVSS)

Fortunately, transient protection

is not expensive. Virtually all

electronic equipment has (or

should have) some level of pro-

tection built in. One commonly-

used protective component is the

MOV (metal oxide varistor) which

clips the excess voltage.

TVSS are applied to provide

additional transient protection.

TVSS are low voltage (600 V)

Fluke Corporation

PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-51

16

In Europe/M-East/Africa (31 40) 2 675 200 or

Fax (31 40) 2 675 222

In Canada (800) 36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2004 Fluke Corporation. All rights reserved.

Printed in U.S.A. 10/2004 2403183 A-US-N Rev A

Fluke. Keeping your world

up and running.

being developed for 230 V/50 Hz

equipment and for adjustable

speed drives. Sensitive equip-

ment should be able to survive

events inside the curve. Events

outside of the curve could require

additional power conditioning

equipment or other remedial

action. A major change in ITIC is

that the ride-through times for

outages as well as the tolerance

for sags have both been

Figure 4. ITIC Curve.

500

400

300

200

140

120

110

100

90

80

70

40

0

Applicable to 120, 120/208, and

120/240 nominal voltages

110 V

90 V

1µs1ms 3 ms 20 ms 0.5 s 10 s Steady

state

0.001 c 0.01 c 0.1 c 0.5 c 1 c 10 c 100 c1000 c

Duration of disturbance in cycles (c) and seconds (s)

Percent of nominal voltage (RMS or peak equivalent)

Voltage-Tolerance

envelope

devices and are tested and certi-

fied to UL 1449. UL 1449 rates

TVSS devices by Grade, Class

and Mode. As an example, the

highest rating for a TVSS would

be Grade A (6000 V, 3000 A),

Class 1 (let-through voltage of

330 V max) and Mode 1 (L-N

suppression). The proper rating

should be chosen based on the

load’s protection needs:

•A lower Grade might result in

a TVSS that lasts one year

instead of ten years. The solid

state components in a TVSS

will themselves deteriorate as

they keep on taking hits from

transients.

•A lower Class might permit too

much let-through voltage that

could damage the load. Class 1

is recommended for switch

mode power supplies.

•A Mode 2 device would pass

transients to ground, where

they could disrupt electronic

circuit operation.

increased. The field trou-

bleshooter must keep in mind

that the profiles are recommen-

dations and that a particular

piece of equipment may or may

not match the profile. Having said

that, the profiles are still useful

because, when recorded events

are plotted against them, they

give a general idea of the voltage

quality at a particular site.