3500/42-09-00 Bently Proximitor Seismic Monitor

Description

v Correcting electrical noise problems can sometimes be a trial-and-error routine. In

troubleshooting noise problems, sometimes it is beneficial to experiment with the shield ground

point location. However, in general, it is usually best to ground the analog input cable shields

as close to the source of the noise as possible, 3500/42-09-00 which is usually at the device (analog source)

end. Cable shield should be grounded at one end only. Also, it is best to keep the length of

stripped cable leads as short as possible to minimize the length of unshielded conductors that

will be exposed to the noisy environment.

3500/42-09-00 Shielding for Analog Input Modules

Generally, the shield for analog input cables should be grounded at the analog source, as shown

in the next figure. However, ground connections for each channel, labeled COM and GND, on

the terminal board are provided for connecting shields at the analog input module if

appropriate. An analog input module’s COM terminals connect to the analog circuit common

in the module. The GND terminals connect to the baseplate (frame ground). The shields may

be connected to either COM or GND.

Technological advances used in electronic circuit design are often aimed at making equipment

smaller, faster, and more sensitive. This effort increases the concern for electrical noise.

Therefore, shielding and grounding are important when 3500/42-09-00 installing a Series 90-30 PLC system.

It is impossible to provide a practical installaton guide that covers all possible application designs.

Sometimes, experimentation with different grounding methods under actual operating conditions is

required to achieve maximum noise rejection. However, this section discusses four shield

grounding examples for analog input modules that have proven effective for most cases.

3500/42-09-00 For an unbalanced source, the ground shield should be connected to the source common or ground

at the source end. If all of the source inputs to this module come from the same location and are

referenced to the same common, all shield grounds should be connected to the same physical

ground point. If using a terminal strip between the analog input module and the field devices

(analog sources), use the method in the following figure for “continuing” each cable shield

using a terminal on the terminal strip. Note that each cable is only 3500/42-09-00 grounded at one end – the

end closer to the field devices (analog sources). Shield connections are shown in bold in the next

figure.

It is usually preferable to ground cable shields at the source end. However, in cases where this may

be difficult or in environments where electrical noise is not a major problem, it may be acceptable

to ground cable shields at the analog input module end. In this case, you would connect them to

one of the module’s GND terminal screws (which is connected to frame ground through an internal

PLC path). This cable grounding method is shown in the next figure in bold lines.

If using the method in the previous example, you may need to improve noise immunity. 3500/42-09-00 To

accomplish this, a conductor may be used to connect the applicable ground terminal on the

module’s terminal board to earth ground. This additional connection will bypass noise around

the module. The next figure shows the same shield grounding scheme as in the previous figure,

but with the addion of the external ground connection just described, shown with bold lines.

NOTES:

1. If the source is floating, connect (–) conductor to Analog Input Module COM

terminal to limit common–mode voltages. Note that common–mode voltage is

limited to 11 volts.

2. If inaccurate readings persist due to noise, the (–) conductor may also be

connected to the Analog Input Module GND terminal.

3500/42-09-00 CAUTION: The Analog Input Module must be the last device in the circuit. When

grounding the (–) return side of the Analog Input Module, the other current sensing

device must be floating and able to withstand a common mode voltage of at least 10

volts, including the noise level.

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