Instrumentation - Grounding/Bonding?

[brightidea]

Hello Everyone!

If you can help out and clear up some confusion I have with grounding and bonding, especially when it comes to instrumentation.

There are many terms that are being used that cause some confusion and I'm sure debate too. Such as, the grounding conductor, grounded conductor, neutral conductor, bonding conductor, signal ground and so on.

When I was an apprentice my code teacher told me the most confusing part of electrical theory for him was/is grounding and bonding. Every time he thought he got a good handle on it something new to his understanding perplexed him. I begin to see what he meant when he mentioned that. Now that I apply myself in the trade, I have more questions then ever before.

What are the big stumpers for you folks?

I am familiar with the term star grounding, and somewhat understand the theory behind it, but in practice it can be confusing, such as the installation of shielded cables.

Signal ground should be isolated at one end to allow for sheath currents to drain, or not allow for currents to circulate and cause interference.

In some cases the shield conductor and the ground wire are bonded together and tie into the panel grounds.

Continous grounds or isolated?

Balanced or Unbalanced ground schemes?

What common practices for instrumentation grounding should be uses or are best used?

Also what about the equipotential plane, how does grounding at one end of a device and leaving the other end isolated work together?

Im installing instrument devices and other power and control equipment in a 230kv and 500kv Transformer Station and wonder how the noise/interference, EMI, affect my installation?

Electro-Static Induction or Electro-Magnetic Induction?
TOUCH/STEP VOLTAGE?

Much of the system has no ground wire through the cables but have metalic sheaths, All enclosures have bonding conductors attached but nothing going back to connect between runs.

A common problem when dealing with old and new wiring.

I hope that some of the more experienced folks could entertain my questions.

Much appreciated
brightidea

 

[dereckbc]

Too many questions for any of us to answer all at once. First you need to sort out some things because you have conflicting terms. Here is a start.

“Balanced or Unbalanced ground schemes?”

Sorry but this does not make any since. I think you mean balanced signal transmission or unbalanced transmission.

Balanced transmission forms are twisted pairs like your telephone, Ethernet, thermo-couples, instrumentation, or twin-lead of an antenna. In this form ground is not used as a reference point, the signal is balanced equally by polarity above and below ground potential. Balanced signal transmission has high rejection of common-mode noise

Unbalanced signal transmission is things like coax or RS-232 where ground is used as part of the transmission path. It is very susceptible to common-mode noise

“Also what about the equipotential plane, how does grounding at one end of a device and leaving the other end isolated work together?”

Again you are confusing two things. In a equipotential ground plane imagine the whole site is built on a sheet of copper. No matter what two points along the plane you measure high frequency impedance, it is always zero. So in a equipotential ground plane design you bond everything to the plane, even both ends of the shields. The reason is because of the low impedance nature of the equipotential ground plane. No matter how much common-mode current is injected in the shield, the noise voltage developed is zero as dictated by ohm’s law I*R, where R=0 and anything times 0 = 0.

“Continous grounds or isolated?”

Complete non-sense. I think you mean isolated or integrated ground planes. Not enough time on my hands to go into this now. Only comment to get you thinking is in a isolated ground plane, everything is a single point ground like a star. For cuurent to flow takes a complete path or loop. If you only have a single point, no current can flow, therfore no voltage in theory.

 

[petersonra]

It is hard to generalize but here are a few of my observations.

Shields are not especially effective or necessary at reducing noise in many cases such as current loops (like 4-20 mA). The circuit impedance is low enough that it is very difficult for noise to be superimposed onto the signal.

High impedance signals are a little different, especially things with millivolt level signals.

IMO, the primary reason many signals are run in shielded cable is tradition.

The old rule of not grounding the shield at both ends has some merit. I think for short runs (perhaps<100 feet) you are probably best off grounding the shield at one end. Longer runs you are probably better off breaking the shield somewhere in the middle and grounding both ends.

In the end your best bet might be to forget the shield altogether and use UTP cable.