hb022 Earthing power conditioners - 4 options
The earthing connections for conditioners installed to isolate noise are very
important.
The unit is designed to isolate the load from the building earth as well as
to provide voltage stabilisation and noise attenuation.
The earth on the protected load may be connected to the safety earth but it
must be realised that noise spikes on the safety earth may cause problems.
This applies to so-called `clean’ or `dedicated’ earths run from the main building
power distribution box.
Ideal installations have the input of the conditioner protected by the safety
earth and a separate clean earth provided
for the critical load.
This can be achieved by an earth rod, water pipe (when suitably tested), or
structure in tall buildings.
A `clean’ earth may be made by employing any of the following:
Earth rod
Most installations can be earthed by driving a 10 mm x 2.5 m long earth rod
within 30 m of the critical load.
This rod is then connected using at least the same size wire as the feeder
circuit.
Water pipe
A water pipe can be used if the resistance to earth is less than 6 Ohms.
Make certain by testing that the water pipe is not isolated by plastic connections,
especially where it leaves the building.
Building structures
When the installation is in a multistorey building it will be necessary to
connect to the structural steel for the clean earth.
Choose a point that is close to the conditioner and bolt a wire to the structure.
Connections to the structure are usually better than running a wire to the basement.
Earth usage
In those cases where the load is partially protected (some peripherals unprotected
by the conditioner) we recommend that the low or pseudo neutral side of the
conditioner output is connected to the clean earth to avoid voltages appearing
between the floating output of the double wound CVT
and the unprotected peripheral.
Further attention should be given to ensure that earth connections are `star-wired’
to the clean earth.
Local permanent wiring regulations should always be observed.
The four major options are as follows:
 |
1 the output of the conditioner is floating with the safety earth
wired through. Advance plug/socket units are like this. Unit fails normal
earth loop impedance testing but is safe to BS 3535. If one output is
fault connected to earth the other becomes hazardous. The conditioner
will work OK. If both outputs are earthed the unit will close down to
a safe condition. Note that some poorly designed SMPS are sensitive to
floating neutrals. |
| 2 hardwired using separate clean earth. This gives good overall noise
performance. Clean earth must test out properly and be isolated from safety
earth. Clean earth should be labelled in UK to 514-7. |
| 3 as 2 but establishment of quasi neutral. Output high is now potentially
hazardous and should be connected via an RCCD if feeding sockets. Common
mode noise may be better than 2 above in some situations. For first time
installations this option gives the most straightforward solution. The
conditioner operates like a new distribution transformer. |
| 4 safety earth wired through quasi neutral. Common mode attenuation
poorer. Installation should use RCCD with sockets.
The output is high-and-low not live-and-neutral so earthing the low output
terminal is entirely compatible with the Regulations. |
part of the AEL technical handbook return to hb000
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