outlets in our laboratories are connected as shown in
figure 1, and this is the same configuration as most domestic
power outlets in your home. Signage on each bench will
indicate which GPO's are earthed. Earthed GPO's mounted on
benches in general laboratory space (Zone 1) are protected
by personnel grade earth leakage circuit breakers. Most
laboratories also have wall-mounted, earthed GPO's used for
cleaning equipment. You must only use the GPO's mounted
on benches as the wall mounted GPO's do not have personnel
grade earth leakage protection.
should also be noted that not all earthed GPO's will have
an indicator lamp, and on those that do, if any of the indication
lamps fail, there will still be power to the GPO once the
corresponding isolation switch is closed.
1: A Double GPO and its internal circuit
GPO's will be the outlets normally used for the vast majority
of laboratory equipment. The other three pin outlet on some
laboratory benches is a non-earthed, isolated, outlet and
these should not be used unless your application has special
requirements for them. The specific circumstances when these
outlets are used are discussed in the section titled Non-earthed,
insulated, standard laboratory equipment will
have its supply plug connected as shown in figure 2 (i.e.
has three pins on its supply plug).
2: Wiring of standard 240VAC plugs
Any equipment that is single
insulated will have its chassis and any external
metal components connected to mains earth via the Earth
pin on the supply plug.
no circumstances should you change or interfere with the
wiring of a standard 240V plug or the internal equipment
wiring. If your application has special requirements, consult
a laboratory staff member. Plugs
should be held by
the body, not the cable when being plugged or unplugged
and equipment should be plugged/unplugged with the
isolation switch off.
Each bench has either one or two isolation circuit breakers located on the distribution board of each laboratory. These circuit breakers provide protection for the building wiring. The breakers are marked by bench number, one breaker controls power to both the earthed and isolated, non-earthed bench outlets, the other controls power to the three phase bench outlets.
certain applications, it is necessary to isolate the equipment
we are using from mains earth and sometimes from the AC mains.
For example, consider
the diagrams shown below in figure 3. In this example the
user is trying to display the waveform across the impedence
Z2 on an oscillosope.
As the 5V supply is earth referenced, placing the reference
of the oscilloscope at the Z2-Z4 junction
will create a short circuit across the equivalent impedence Zx
In figure 3 (a) the reference from the ocilloscope's probe
is connected to earth via the Earthed
GPO. Figure 3 (b) shows how the waveform can be
displayed by using a Non-Earthed
Isolated GPO to power
3: Example application where it is necessary to
use a Non-Earthed Isolated GPO
outlets, as when using an isolation transformeras in our laboratory
benches have their active and neutral supplied by the secondary
of an isolation transformer (i.e. there is no return path
for current from any of the two secondary terminals to mains
earth), with the earth pin of the GPO not connected or floating.
single insulated equipment
supplied from a non-earthed, isolated outlet will be floating
(not referenced to mains earth) and may be referenced arbitrarily.
If equipment is supplied
from a non-earthed, isolated outlet, the equipment earth
may be connected
to an arbitrary potential, and consequently, the equipment
chassis and any other exposed metal components will also
be at that potential.If this
potential is a hazardous mains referenced, or differential
potential, the risk of electrocution by contact with exposed
metal components on the equipment chassis is increased. Great
care must be taken when floating equipment, and in general,
undergraduate students are not permitted to float equipment
to hazardous potentials without supervision.
4 shows an example where the chassis of an oscilloscope is
at a dangerous potential. As the reference for the second
oscilloscope is connected directly to the 200V output of
an earth referenced supply the chassis of this oscilloscope
will also be at 200V with reference to earth. Under these
circumstances extreme caution must be taken to avoid contact
with the chassis of the second oscilloscope.
4: Example where equipment chassis is at a dangerous potential
of our laboratory non-earthed, isolated outlets use double
GPO sockets. Referring to figure 1, it should be noted that
although mains earth is not connected to the GPO, both earth
pins on the GPO are connected internally and will necessarily
be at the same potential once an instrument is connected. Particular
attention must be paid to referencing when using multiple instruments
in non-earthed, isolated outlets.