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Subject: Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
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It is not uncommon to see hi-fi equipment with the earth (ground)=20
disconnected from one piece of equipment or another, usually to prevent =
a hum=20
loop which ruins the listening experience. However, there is nothing =
quite like=20
being electrocuted to really ruin the experience should something =
go=20
wrong!
Electrocution First, a quick word on electrocution. It is =
not fun,=20
and electricity kills a great many people worldwide every year. A =
current of=20
50mA (barely enough to make a low wattage lamp even glow) is sufficient =
to send=20
your heart into a state called 'ventricular fibrillation', where the =
heart=20
muscles are all working out of synchronisation with each other. Little =
or no=20
blood is pumped, and you will die within about 3 minutes unless help is=20
immediately at hand.
Sometimes (but less often), your heart will simply stop. If this =
happens, it=20
is possible that with external heart massage that it might re-start, and =
occasionally it might even re-start by itself - rare, but it can happen. =
The=20
result of fatal electrocution is that you will no longer be able to =
enjoy the=20
hi-fi that you have spent so much time and money putting together, and =
all other=20
earthly activities are similarly curtailed .
This article has been prompted by many e-mails I have received asking =
about=20
hum, earthing and what should be done to ensure that equipment is safe, =
and does=20
not hum. There are other causes of hum in a sound system other than =
electrical=20
(safety) earthing issues, but I will contain this particular article to =
the=20
basic issues of safety and eliminating hum loops while maintaining a =
high degree=20
of safety.
The regulations regarding safety earth connections vary =
from one=20
country to the next, and I do not have the details for each case. =
This=20
article is general, and if unsure, you should consult the =
appropriate=20
electrical supply authority in your country to obtain the rules =
that apply=20
to you.
I have used the terminology I am familiar with - =
the live=20
conductor is called the 'active' and the return conductor is =
called=20
'neutral'. The safety conductor is called 'earth' or sometimes =
'earth=20
ground' (particularly in the US). These terms differ, so make sure =
that=20
you know what they are called where you =
live.
How The Safety Earth Works=20
The basic idea of an electrical safety earth (or ground) is pretty =
much the=20
same everywhere, but the details can vary widely. The case (chassis) of =
the=20
equipment (and except for special situations, the internal electronics) =
is=20
connected to an earth pin on the mains outlet. This is then connected =
through=20
the house wiring and switchboard to an electrically solid earth point, =
which is=20
commonly a copper water pipe (no longer allowed in Australia or NZ), or =
an=20
approved earth stake buried deep into the ground.=20
In some systems used elsewhere, the earth wire is separate back to =
the=20
distribution transformer, and in others the neutral is also the earth up =
until=20
the household switchboard. Australia and New Zealand use the 'Multiple =
Earth=20
Neutral' (MEN) system, where there is a bond between neutral and earth =
at each=20
household (or unit complex) main switchboard. The maintains the lowest =
possible=20
impedance for the safety earth. Other countries have different =
regulations and=20
systems - look it up for your location or ask an electrician if =
unsure.
Should a fault develop within the equipment that causes the active =
(live)=20
conductor to come into contact with the chassis, the fault current will =
flow to=20
earth, and the equipment or main switchboard fuse or circuit breaker =
will blow.=20
This protects the user from electric shock, bypassing the dangerous =
current=20
directly to earth, rather than through the body of the unsuspecting poor =
bastard=20
who just touched it. If this experience does not kill, it will =
invariably enrich=20
the vocabulary.
Earth leakage circuit breakers (RCD - residual current detectors - =
see below)=20
measure the current in the active and neutral conductors. If these =
differ by=20
more than a few milliamps, the circuit is disconnected. The principle is =
simple=20
- if the current in the two wires differs, some of it must be going =
somewhere=20
that is undesirable, so the supply is interrupted almost instantly. =
While these=20
are mandatory in some countries (or under some circumstances), it is =
best not to=20
rely on any advanced technique, but provide a system that is =
intrinsically safe=20
- this is extremely difficult in reality.
There are exceptions to the basic earthed equipment method of =
protection.=20
Some equipment is designated 'Double Insulated', and usually has a =
symbol of two=20
concentric squares that indicates that the equipment is double =
insulated, and=20
that an earth connection is not needed (or in some cases must not =
be=20
used). The common plug-pack (wall-wart) power supply is nearly always =
double=20
insulated, and such equipment has reinforced insulation, designed to =
ensure that=20
it is not possible for the live AC connection to connect to the =
secondary=20
electronics in any event - including a complete meltdown. The electrical =
safety=20
tests to verify that a product meets the Double Insulation standards are =
rigorous and expensive, and are very difficult to meet with high powered =
equipment, and even more so when the equipment has a metal case. Nearly =
all=20
power amplifiers (for example) are not double insulated, and =
require an=20
earth connection.
Colour Codes=20
The common colour codes for mains wiring are shown in Table 1, below. =
The=20
Active (or Line) is the 'hot' conductor, and carries the full AC supply =
voltage.=20
The Neutral conductor is not live, but is intended to be the return path =
for all=20
current in the active lead. The neutral is always considered to be a =
'live'=20
conductor and must be insulated accordingly, and it must not be used for =
anything other than the return path for current from the active. =
Although this=20
has caused great confusion to a great many people, it is sensible and =
logical (I=20
will not go into the reasons here). The safety earth (or ground) =
conductor is=20
intended to provide protection against electrocution, and where fitted, =
must not=20
be disconnected.
Conductor
IEC
US
Alternative
Active (Line)
Brown
Black
Red
Neutral
Blue
White
Black
Earth
Green/Yellow
Green
Green
Table 1 - =
Common Mains=20
Colour Codes
These colour codes are not standardised, and some variations may be =
found in=20
different countries. The column headed 'Alternative' refers to an old =
code that=20
used to be used in Australia and several other countries prior to the =
IEC codes=20
being adopted. The one common theme of these codes is that they have =
been=20
designed so that colour-blind people will not get the wires mixed up. =
The use of=20
green with yellow stripes for the earth makes this even more secure. I =
have no=20
information on the history of the determination of the colours used, but =
it is=20
of little consequence since we can't change it.=20
Note that in the US, the neutral is sometimes referred to as the=20
groundED conductor, and earth/ earth ground called the =
groundING=20
conductor. These terms are not intuitive and are easily mixed up if you =
don't=20
understand the difference. Make sure that you fully understand =
all terms=20
that are used where you live.
Earth Loops/ Ground Loops=20
Figure 1 shows a typical connection of two hi-fi components, and =
includes the=20
house wiring and main earthing point. As can be seen, there is a loop =
(indicated=20
by the dotted line), which includes the interconnect cable, power leads, =
and a=20
small part of the house wiring. Such loops are a major cause of hum in =
systems,=20
and it is not uncommon for people to remove an earth wire from one or =
the other=20
mains connectors to break the loop and stop the hum. The situation is =
much worse=20
if different wall outlets are used for different parts of the sound =
system. In=20
this case, the loop may extend all the way back to the main switchboard, =
making=20
it longer, and more likely to have a significant voltage between =
individual=20
earth connections.
Figure 1 - The Formation of an Earth Loop
Also note that the neutral (return) conductor is attached to earth at =
the=20
main switchboard. This is called the MEN system and is standard in =
Australia=20
and some other countries, but might not be the case where you live. =
Check with=20
an electrician who can tell you how this is done (if you really want to=20
know).
What happens if the amplifier develops an electrical fault that =
allows the=20
live AC conductor to come into contact with the chassis? The current =
will flow=20
from the chassis, through the earth connection, and the fuse/ circuit =
breaker=20
will blow in the switchboard (or in the equipment if a mains fuse is=20
fitted).
Should the safety earth be disconnected from the power amp (for =
example), if=20
a fault occurs in the amp, the only earth return is now via the =
interconnects=20
(assuming that the source is earthed). Interconnects are not designed to =
withstand the fault current that can occur with a major electrical =
fault, and=20
may disintegrate before the fuse. You now have a live chassis on the =
amplifier -=20
just waiting for someone to touch it and possibly die!
Residual Current Detectors=20
Many new installations use a safety switch, specifically an 'Earth =
Leakage'=20
or 'Residual Current' detector (aka GFI - ground fault interrupter, =
etc.), a=20
device that will disconnect the AC supply if the current flowing in the =
active=20
(live) conductor is not exactly matched by that in the neutral. Any =
imbalance=20
means that current is going somewhere it should not be, and the device =
will=20
trip.
These safety circuit breakers are very fast acting, and have saved =
many lives=20
since their introduction. The 50mA that will kill you is detected by the =
breaker, and the power is disconnected - fast! Most of these type of =
breakers=20
will operate on as little as 20mA, so you are not only protected against =
major=20
faults, but also against excessive AC leakage caused by faulty =
insulation or=20
moisture.
This does not mean that you can now go around disconnecting earth =
connections=20
to stop hum - the safety devices that may be fitted to your house wiring =
are=20
designed to trigger on a fault before you find it the hard way. =
In many=20
countries, it is illegal to tamper with electrical (mains) wiring unless =
you are=20
licensed - but in all countries, if it can be proven that you =
disconnected an=20
earth that allowed a fault to kill someone else, you are liable, and =
may be=20
subject to criminal charges ! Is that scary?
What Causes Earth (Ground) Loops?=20
It is generally accepted that an earth loop conducts a current from =
one piece=20
of equipment to the next, and imposes a voltage across the connection. A =
good=20
question is where does the current come from, and why doesn't it trip =
the safety=20
switch? Contrary to common belief, earth loops are not caused by =
leakage=20
current or some other mysterious current that flows in the earth lead =
back to=20
the switchboard. If this were the case, it would have to be coming from =
an=20
active connection via a leakage path, and this would trip the safety =
switch=20
instantly.=20
The loop is mostly entirely local, and (again) contrary to some =
claims,=20
connecting equipment to separate mains outlets will almost certainly =
make the=20
situation much worse. Current in the local loop is created by the stray =
magnetic=20
field of transformers in the connected equipment. Traditional (EI) =
laminated=20
transformers are almost always worse in this respect than toroidal =
transformers,=20
but all mains frequency transformers are capable of generating a =
circulating=20
current if given the opportunity. These currents are made worse if there =
is=20
metal chassis work in close proximity to the transformer laminations. =
Thick=20
panels simply mean lower resistance, so a higher current flows for a =
given=20
induced voltage.=20
Another source is a signal lead running parallel to a mains power =
lead.=20
Although the conductors of mains leads are twisted, the twist is usually =
fairly=20
basic, so balancing of the magnetic field is rather poor compared to the =
tight=20
twist of Cat-5 communications cable for example. The magnetic coupling =
is poor,=20
and the greatest problems are likely to be caused by capacitive =
coupling. Since=20
this favours high frequency noise, the sound is completely different =
from an=20
earth loop, and it's a good idea to try to familiarise yourself with the =
different sounds made by the various issues that may plague hi-fi =
setups. If=20
signal cables and mains wiring must cross each other, ensure they cross =
at right=20
angles, and if possible separate the two as far as practicable. =
Capacitive=20
coupling can also be an issue with transformer windings, where mains =
noise is=20
coupled through to the secondary by inter-winding capacitance, or from =
Y-Class=20
caps from mains to chassis.=20
It is not at all uncommon that multiple earth loops may exist, but in =
the=20
vast majority of cases a transformer is the root cause of the problem. =
The loop=20
created by the mains safety earth and the various interconnects can be =
quite=20
large, and it may not seem possible that cables so far from a =
transformer could=20
possibly generate enough current to cause a problem. However, the cables =
might=20
well be separated, but what about the equipment chassis? Any metal panel =
that=20
passes close to the transformer becomes part of the problem too, =
depending on=20
how the internal earthing connection is arranged. It's not uncommon to =
see the=20
mains safety earth connected near the mains inlet, and the signal earth=20
connected somewhere else on the chassis. In isolation, this will never =
cause a=20
problem. Once the equipment is connected to something else that's also =
earthed,=20
an instant earth loop is created.=20
While disconnecting the mains earth from one of the offending pieces =
of kit=20
may well break the loop, it also renders the setup unsafe if there is an =
internal fault. In some countries, it may be illegal to disconnect a =
safety=20
earth, and if someone is killed or injured you may be held liable.
Figure 2 - Transformer Induced Earth Loop Current =
Waveform
The waveform shown above was not simulated. It was captured on a PC =
based=20
oscilloscope, using a single loop of thin wire (loosely) around the =
outside of=20
an E-I core transformer. No special attempt was made to optimise the =
signal, and=20
the loop was terminated by a resistor of 0.22 ohms. The voltage was =
changed only=20
very slightly regardless of whether the resistor was connected or not, =
showing=20
that it is not unreasonable to expect that the current may be very large =
indeed=20
if the impedance of the loop is low enough. Note that the primary =
frequency is=20
50Hz, but the waveform shows that there is a very high level of 150Hz =
... the=20
third harmonic.=20
Remember that any metalwork, including that of another piece of =
equipment=20
sitting above that which uses the transformer, becomes part of this =
loop.=20
Increasing the size (effective diameter) of the loop does reduce the =
problem=20
slightly, but the larger loop may also be more sensitive and need less =
magnetic=20
flux to generate a potentially troublesome voltage and current.=20
Although the measured voltage of the waveform in Figure 2 is only =
about=20
20mV, compare this with the signal voltage at a typical listening level. =
Assuming speakers at around 90dB/W/m and a power amp gain of 27dB, the =
hum is=20
only 15.7dB below a listening level of 1W (90dB SPL at 1 metre). This =
will be=20
very audible indeed.
Main Earth Connection=20
For those who build amps (as is the case with many of the readers of =
these=20
pages), a common question is "How should I connect the mains safety =
earth to the=20
chassis?". As I stated above, the regulations change from one country to =
the=20
next, but the principles are the same. Figure 3 shows a view of the =
basic=20
connection, which is very safe. The lug used should be an approved earth =
lug (or=20
one that meets any standards that exist where you live). Most are =
crimped, but=20
soldering ensures the most reliable connection for safety.
Figure 3 - Safe Method Of Connecting The Safety =
Earth
Any paint (or anodising, in an aluminium chassis) must be scraped =
away to=20
expose bare metal, and the tooth washer ensures that there is a good =
'bite' into=20
the metal itself. The use of two nuts is strongly recommended, since the =
second=20
one acts as a locknut, and prevents the first nut from loosening. The =
flat=20
washers shown are optional, but highly recommended. They may be a =
requirement in=20
some countries.
Do not use the earth connection as mounting for any other panel or =
component=20
- it must be dedicated to the task of providing a safety earth point. If =
a=20
component mounting bolt is used, at some stage it may be disconnected by =
a=20
service (or other) person, which means that the apparatus is unsafe =
until=20
everything is (hopefully) put back where it belongs - this does not =
always=20
happen.
Make sure that the electrical connection between metal panels is also =
very=20
well made. Some chassis are available in a kit form, and when screwed =
together,=20
may not make good electrical contact with each other. Should the mains =
come in=20
contact with a panel that has a flaky connection with the one that is =
earthed=20
properly, the same potential for disaster is still present. All exposed =
metal=20
must be properly and securely earthed.
The internal electronics of an amplifier should also be earthed, but =
now we=20
have the problem of the hum loop again. There are two possibilities here =
...
Don't earth the internal electronics, or use a simple 'loop =
breaker'=20
circuit to allow the case to act as a shield for radio frequency =
interference,=20
but no solid connection is made (this is a common approach). This =
provides=20
protection should there be a failure from the incoming mains to =
chassis, but=20
provides none at all if the transformer were to develop a fault =
between=20
primary and secondary windings. Such faults are uncommon, but they can =
(and=20
do) occur.
Use a high current loop breaker circuit, ensuring that even major =
fault=20
currents will be bypassed to the safety earth conductor. Such a =
circuit was=20
described as a part of the 100W Guitar Amp project, but is shown =
again below.=20
Be warned that this circuit (while safe) may not be legal where you=20
live.
Use Of Loop Breaker Circuits=20
While very effective (and safe), as mentioned above such a circuit =
might not=20
be legal where you live. If this is the case and hum is causing you =
grief, the=20
use of balanced interconnects might solve the problem - but at =
some cost,=20
and will require balancing circuitry at each end of all the =
interconnects. While=20
not a panacea, this is the approach taken in all professional equipment, =
and is=20
usually highly effective, allowing all safety earth connections to =
remain=20
exactly as they are to prevent electrocution of the artists or road =
crew.=20
Suitable circuits for home (or professional) use are shown in the =
projects=20
section. =20
Note that there is an ongoing debate about the proper connection of =
pin 1 of=20
all XLR connectors, and if not done appropriately for the equipment, the =
"pin 1=20
problem" may either defeat any benefit from balancing or even make =
matters=20
worse. In nearly all cases, transformers are more effective than =
electronically=20
balanced circuits, but good ones come at high cost, and cheap ones may =
seriously=20
affect the frequency response of the equipment.
Figure 4 - A High Current Safety Loop Breaker =
Circuit
I have simply shown all internal electronics as a box, with the only=20
connection to the loop breaker being the zero volt line. This is most =
commonly=20
taken directly from the centre tap of the main amplifier filter =
capacitors, but=20
should always be connected to a point where there is high current wiring =
back to=20
the transformer. It is the transformer that provides isolation from the =
mains,=20
and the possibility of an internal transformer fault must be catered =
for.=20
Ideally, the mains earth connection and the loop-breaker's earth end =
should=20
connect to the same point on the chassis (as shown). Depending on the =
installed=20
transformer, there may be a significant circulating current within the =
chassis=20
itself.
The only exception is if a double insulated mains transformer is =
used, but=20
these are rare. Should the transformer be of 'conventional' construction =
(not a=20
toroidal), then the transformer body - the steel core - must be =
connected to=20
chassis directly. Do not use any loop breaker circuit to isolate the=20
transformer core, as it is unnecessary and dangerous to do so.
The loop breaker works by adding a resistance in the earth return =
circuit.=20
This reduces circulating loop currents to a very small value, and thus =
breaks=20
the loop. The capacitor in parallel ensures that the electronics are =
connected=20
to the chassis for radio frequency signals, and helps to prevent radio =
frequency=20
interference. Finally, the diode bridge provides the path for fault =
currents.=20
The use of a large chassis mounting (35A) type is suggested, since this =
will be=20
able to handle the possibly very high fault currents that may occur =
without=20
becoming open circuit. Note the way the bridge is wired, with the two AC =
terminals shorted, and the two DC terminals shorted. Other connection=20
possibilities are dangerous, and must be avoided.
In the event of a major fault, one (or more) of the diodes in the =
bridge will=20
possibly fail. Semiconductors (nearly) always fail as short circuit, and =
only=20
become open circuited if the fault current continues and 'blows' the=20
interconnecting wires. High current bridge rectifiers have very solid =
conductors=20
throughout, and open circuit diodes are very rare (I have never seen a =
high=20
power bridge go open circuit - so far at least). Use of the bridge means =
that=20
there are two diodes in parallel for fault current of either polarity, =
so the=20
likelihood of failure (to protect) is very small indeed.
When a loop breaker is used, it is vitally important that all input =
and=20
output connectors are insulated from the case. If not, they will =
instantly=20
defeat the loop breaker by providing a direct connection from the zero =
Volt=20
point to chassis, and no benefit is obtained. (Electricity has an =
annoying - but=20
perfectly logical - tendency to travel along the path of least =
resistance, and a=20
direct short circuit will always have less resistance than the loop=20
breaker.)
It is not uncommon to have an induced voltage of perhaps 1V RMS =
between the=20
earth connections of power outlets that are wired separately back to the =
switchboard. This small voltage, with a total resistance of perhaps =
0.2-0.5 Ohm,=20
will cause a loop current of 2 to 5 Amps, all of which flows in the =
shield of=20
the interconnect. This is sufficient to cause a voltage difference =
across the=20
interconnect, which the amplifier cannot differentiate from the wanted =
signal.=20
By breaking the loop with the 10 Ohm resistor, the current is now less =
than=20
200mA, and the voltage across the interconnect will be very much =
smaller,=20
reducing the hum to the point where it should no longer be audible.=20
Never route an earth wire to the main (star) earthing point on a =
chassis in=20
such a way that it forms a partial (or full) turn around a transformer. =
It is=20
better to relocate either the star earth point or the transformer to =
ensure that=20
no earth conductor can create a partial turn. There may often be =
conflicting=20
requirements, but there is usually no reason that proper earthing for =
minimum=20
hum and maximum safety should be mutually exclusive. Both are important, =
and=20
both must be accommodated in the final design.
An earth loop will typically inject either a 50Hz or 60Hz =
hum=20
into the signal, or in the (common) case of a transformer induced =
current,=20
a somewhat mangled mains frequency as shown in Figure 2 - if you =
have a=20
100Hz or 120Hz buzz (which generally has a hard edge to the =
sound), you=20
have done something wrong in the wiring of the power supply, and =
the=20
techniques described here will not help. Refer to the article on =
power supply =
wiring.
Mains Filters=20
The core of the transformer (only for C-core or EI types - it's not=20
accessible with toroidal transformers) should also connect to the =
chassis and=20
mains earth. In the unlikely event of a primary to core leakage or short =
circuit, the current will be diverted to the protective earth and will =
trip the=20
safety switch or circuit breaker. In some cases, a transformer may be =
fitted=20
with an electrostatic shield, but these are lamentably uncommon in hi-fi =
transformers. Where provided, these too must be connected directly to =
the main=20
earth point, and not via a loop breaker (if used).
The purpose of the electrostatic shield is to intercept (and earth) =
any=20
interference on the incoming mains. It does this by preventing any =
signal from=20
being capacitively coupled from the primary to secondary windings, so =
the only=20
form of coupling in the transformer is via the magnetic field in the =
transformer=20
core. Most mains transformers have relatively poor high frequency =
response and=20
this helps to further reduce interfering signals.
This can dramatically reduce extraneous noises (clicks, pops, =
whirring=20
sounds, etc.) that might get into the system via the house and supply =
company=20
wiring. This has great potential to pick up noise, since there may be 50 =
to=20
100km (or more) of cable (including high voltage feeders and =
substations)=20
involved between your amplifier and the generating plant.
In some cases, a mains filter might be fitted to amplifiers or other=20
equipment (such as specially designed mains leads or 'black boxes') to =
reduce=20
any interference. Where fitted, if an earth connection is provided, it =
must be=20
connected to the safety earth and chassis - never to the amp's zero volt =
line.=20
Typical filters will use Metal Oxide Varistors (MOVs) to cut off any =
high=20
voltage spikes, and a capacitor and inductor network to filter out =
anything that=20
is not at the mains frequency.
A true 50Hz (or 60Hz) tuned filter will be a large unit indeed, so =
most line=20
filters only work at frequencies above a few kHz. This is generally =
enough to=20
get rid of most interference, since a well designed power supply should =
be able=20
to filter out the majority of noise from the mains. Mains filters =
usually use=20
the mains earth as a reference, so it must be present for the units to =
work=20
correctly. Not using the safety earth as a reference is extremely =
dangerous,=20
since the filters may have capacitors that can (and do) become short =
circuited=20
if a high voltage spike manages to get through and punctures the =
insulation.
Conclusion=20
Electrical safety cannot be over emphasised. Hum is damn annoying, =
and=20
everyone wants it gone. There is no good reason to sacrifice one for the =
other,=20
since safety and hum-free operation can peacefully co-exist with care =
and the=20
right techniques. Use of a separate earth stake just for hi-fi equipment =
is=20
probably unlawful in most countries, as the integrity of the safety =
earth may be=20
suspect at best, useless at worst.
As I have said several times, make sure that you find out the legal=20
requirements in your country, and don't do anything that places you at =
risk -=20
either from electrocution or legal liability. Neither is likely to be a =
pleasant=20
experience.
Where the mains is noisy (apparently a common occurrence in the US), =
use of a=20
dedicated mains filter may be useful to prevent mains noise from =
entering the=20
system. This will generally be unnecessary if the supply is well =
designed=20
(especially if an electrostatic shield is used on the transformer), but =
this is=20
often the exception, rather than the rule.
The use of 'specialty' mains leads (unless fitted with a proper =
filter which=20
will be in the form of a box in line with the cable) is unlikely to =
solve the=20
problem - regardless of claims made by the manufacturers or reviewers =
(see The Truth About Cables,=20
Interconnects and Audio in General for my comments on these - this =
article=20
made a lot of audiophiles very unhappy, but advertising hype does =
not=20
negate the laws of physics).=20
The (relatively) recent trend to use switchmode power supplies in =
consumer=20
equipment, along with double insulation, has created new problems. All =
SMPS use=20
small (and allegedly) 'fail-safe' Y-Class capacitors to the chassis, =
which is=20
not earthed. Use of these caps means that the chassis floats at roughly =
half the=20
mains voltage, but the impedance is very high. This poses two risks =
...
Equipment input circuits may be damaged if double insulated =
appliances=20
with an SMPS are connected while switched on. This is covered =
elsewhere on the=20
ESP site. Such failures are the result of (typically) half mains =
voltage=20
being present on the chassis (and therefore the internal circuitry).=20
Connection to earthed equipment may cause a large instantaneous =
current to=20
flow.
Switchmode supply noise and any high frequency noise on the mains =
now=20
flows in the shield of the interconnect. This is not really an earth =
loop as=20
such, and the result is more likely to be a harsh (grating) hissing =
sound. It=20
is quite distinctly different from normal thermal noise, and is also =
more=20
intrusive.
It might be possible to reduce this noise by installing a =
heavy earth=20
strap that joins each chassis. Strictly speaking, this may be completely =
illegal, but the rules for double insulated appliances in many countries =
are=20
often stupid, and fail to address reality. Almost all modern systems =
will have a=20
mixture of earthed and double insulated equipment, and any rule that =
states (for=20
example) that "double insulated appliances must not be earthed" is =
instantly=20
broken when the interconnects are installed. Needless to say, without =
the=20
interconnects there is no point having the gear in the first place, =
because=20
there's often no other way to get the signal from one unit to the other. =
Optical=20
fibre is one method of course, and completely eliminates any possibility =
of an=20
earth loop. This is not always a viable option.
=20
Main=20
Index Articles=20
Index
Copyright Notice. =
This=20
article, including but not limited to all text and diagrams, is =
the=20
intellectual property of Rod Elliott, and is Copyright =C2=A9 =
1999.=20
Reproduction or re-publication by any means whatsoever, whether=20
electronic, mechanical or electro- mechanical, is strictly =
prohibited=20
under International Copyright laws. The author (Rod Elliott) =
grants the=20
reader the right to use this information for personal use only, =
and=20
further allows that one (1) copy may be made for reference. =
Commercial use=20
is prohibited without express written authorisation from Rod=20
Elliott.
Page created and copyright =C2=A9 30 Dec 1999./ =
Updated Dec=20
2014