Isolating Instruments from Electrical Noise

The most effective method of noise isolation is distance between noise source and all wiring. If this is not possible then much more detailed information on the circuitry and noise sources must be known to make any evaluation.

I understand the most effective means is physical segregation. This has of course been outlined to my client. However I am instructed that to undertake such a task is outside of budgetary confines. The distribution is fairly simplistic and on a very small scale, (less than 50 instruments). Would Baluns and small DC capacitor filters be of any substancial benefit?? Are you aware of any commercially available products?

Now, now. Play nicely.

'allo 'allo, are you really the gendarme ?

Wow! I wish I had another GA vote. That was as cogent an explanation, absent the maths, as I think I've seen with regard to noise summations. I'm gonna swipe the Do Re Mi bit for my own use.

I agree with TVP45; a very clear description of the larger problems involved in this investigation/discovery process. GA.

Your quest to become an instructor seems to be on path. Now, if you can only get the little brats to pay attention...

What TVP and Doorman said....I added a GA. Thanks for the help - and the rose, I'm getting quite a collection!

My background includes "real" noise analysis (the stuff you hear, not this silly only-in-the-wires stuff ) so your explanation makes perfect sense! I usually use the swamp draining analgy, but I like yours a lot.

When (if, given my lab area is being removed at the end of the month ) I get to do some initial measurements, I'll start a thread to ask for suggestions. Briefly, the loom is an addition on a 24V vehicle and we think that a sensor heater (24V) is interfering on a 5V signal line. Until I'm allowed to hire some suitable equipment (and learn more about electrical/electronic noise), I can't give much more detail. An EMC expert friend suggested introducing a ground loop of sufficient size to dampen the noise....if I could be sure of good connections, I could use the vehicle chassis. Trouble there is the vehicles are customers' vehicles and I know that maintenance is not high on their list of priorities.

You mentioned one of the voodoo phrases of electronic noise reduction, a ground loop.

A ground loop is a wire that has multiple paths to ground. In theory one never wants to deliberately fabricate a ground loop because that loop becomes one turn of an air core transformer. However, occasionally one can find a deliberate ground loop configuration that cancels the power distribution noise instead of adding to it.

<Chuckle> I admit I looked at him cockled when he suggested it. Apparently, he found it was a technique that worked in his fixed installations configurations. He said he'd found it acted in that situation as a very large ground plane (which it what I should have typed earlier).

He likened it to what happens to Ocean waves - amplitude grows in shallow water and reduces in deep water - providing a large ground plane helps reduce the noise amplitude. He found that with his equipment, the "plane" worked if it was a large diameter wire that surrounded the equipment.

No good deed ever goes unpunished. One of the best GAs I've seen in a long while.

Change your Client.

How is the noise manifesting itself?

How does your client describe the problem?

the client describes the problem as manifesting itself in "errattic process trends, co-enciding with motor starting". I suspect transcient current in earth is superimposing on analouge dc feedback from instruments. But that's my best guess from the information they have provided.

Transient current in the Earth conductor suggests to me that the motor equipment insulation is failing, which means the Client has got bigger problems than just the instrumentation. What tests have you done to check-out this guess, and can we stop playing "20 questions", please? You'll get a lot better response from this forum when you give all the information at the original posting. And a quicker one, too, which may or may not be important to you

Trouble at motor start means...

1. Coupling to instrument cabling/ground due to contact sparking at motor contactor close (bouncing)- coupling is capacitative via motor winding/ground and motor cable/ground or motor cable to instrument cable capacitance. There is an enormous difference in coupling between cables with insulation touching and with a lot of air between them. You do not say how big the motors are or if 1 or 3 phase. Capacitors at the point where current is broken can reduce the spikes that is at the motor starter. Capacitor must give an alternative path round the point where current is broken
2. Magnetic coupling. Motor starting current is about 6 times running current, plus a large multiplier, maybe 10 for inrush (saturation) current. Bad news, magnetic shielding is not practical. The only way to reduce coupling is keep the distance between go and return paths minimal (minimum transmit and receive loop areas) and move motor and instrument loops apart. Sometimes the transmit loop can be made perpendicular to receive loop. Alas, when people do not think about this before building, the worst combination exists.
   

Has this problem only recently manifested itself, or has it been lingering in the background until an unsuspecting newguy showed up?

If it is an existing problem, who signed off on it and why?

If it is new problem, what changes have occurred?

New equipment, new wiring, new instruments, new operator, clients' unemployed nephew/brother-in-law been in to rewire the plant?

Does anyone smell smoke, get shocked, change operating procedures, new employee?

I think you are going to have to pack up your tool bag, get a thermos or two of coffee, a case of Cheetos and pay them a visit.

From my prevous experience, the only way to get to a solution is to actually go and measure what's going on. Get a consultant involved.

I'm a mechie being tasked with solving an EMC problem on one of our products and was hoping for some tips here, but it seems I'm further along than you are.

Have you visited the site yet and measured these signals for yourself? We all know anecdotal eveidence can be misleading...

Have you tried twisting the wires?

Problem solved. I reviewed a recent thermographic survey of the main distribution board and carried out ductor testing to confirm. Several high resistance joints exist on the multiple earthed neutral link, and is causing an irregularly high potential between earth and neutral. Which subsequently is causing a bad 0v refernce on the instrument distribution board. some severe corrosion of the earth mat is not helping the situation.

Well I guess that if the customer is happy then things are good.

I'd be apprehensive of main distribution connections that are mysteriously corroding. I'm particularly not happy with high potentials between neutral and ground. The only current going between neutral and ground should be fault currents and a small amount of RF return current. It sounds like some equipment is using the ground lead for a return path instead of the neutral. By reconnecting the loose power distribution neutral to ground wire you're hiding instead of solving the problem.

I agree. It needs further investigation to find what equipment has a neutral breakdown. However, I should have specified that this is a 30 year old problem inherit to this plant and this is the first time it's been looked at. I should also mention my "consulting" is more of a hobby and is free of charge, so I'm not trying to claim your expertise as my own and profit from it. It's more a curiosity, and an opportunity for me to learn. Honestly, if my questions are vague, unfortunately it's not an intentional omission. I'm trying to generate the right questions to direct me to make the right queries. Apologies for the short temper.

Not a problem, we've all had days that our tempers run short.

One thing about your "hobby". Regardless of you getting paid for your work, in many places working on power distribution wiring imposes a liability for any repercussions that might happen. It will not matter who worked on what or how old is the rest of the wiring. You worked on it and do not have a license or formal training to support your opinion that you did everything correctly. If anything ever goes electrically wrong you might lose everything you own.

Thanks. I appreciate the concern. I'm actually a licensed technician that works for a consulting firm in the oil and gas industry. When I'm not contractually engaged with a client I offer my advice to clients that have technical queries. My advice comes with a very transparent disclaimer. " I will provide my educated guess, if I can't help you I will refer you to an ACTUAL expert". I don't encourage people to act on my advice without seeking a second opinion.

What you say is so true, Redfred. I have been working on wiring most all my life. I've worked with licensed electricians, learned from them.

In the name of 'maintenance' I have rewired control rooms, installed new equipment, and fixed everything thrown at me. Usually I was the most qualified person around, and I do everything as close to code as possible.

Most businesses don't want to take the time or expense to bring in a licensed electrician everything something electric goes wrong. It's just not feasible. They let maintenance handle it.

I have replaced breaker boxes poorly situated so water ran into them. My replacements are dry and safer than the original install. The 'licensed' electrician just wanted to get done and get out. I see that way too much.

Yes, there is responsibility, but I feel that if I do my best, things are not likely to go wrong. The alternative is nothing gets done, and the situation stagnates.

I've had a lot of success using analog isolation modules like ADVANTECH's ADAM 3014.

They fully isolate input from output and power supply and can convert signal types (4-20mA to 0-10V etc).

I always keep a few in the draw to solve difficult earthing/noise problems.

I haven't heard of that particular module you referred to. However I imagine they are very similar to the barriers used in this instance, although they are used to meet the intrinsic safety requirements for explosive atmospheres rather than isolation. They are of a zener type. Perhaps a galvanic isolator or opto-coupler would be more appropriate. Thanks for the suggestion.

I may be worrying unncessarily, but the barriers you fitted are approved under whichever explosive atmosphere regulations apply in Australia?

Do you use European regulations in Australia, write your own, or use Japanese ones? That's not as daft a question as it sounds. Hong Kong uses some European regulations.

Yes you're worrying unneccesarily. Part of my job is to ensure the hazardous area compliance of equipment I work on, to Australian standards. The australian standards shares majority of it's criteria with it's european equivalent. BASEFA is one of the major certifying bodies of equipment used in australia.

Nearly all sensitive instruments require leads to get to the measurement points. Most leads and probes are not as well shielded as you might hope they are. I have significantly reduced instrument noise in the past by wrapping all leads with copper-clad tape, and grounding one end. This will not help with power supply noise, but will help with radiated signals in the environment.

Grounding both ends will give even more reduction. Properly terminated shielding on wiring is an excellent way to protect against emitted/radiated noise.

[see: I did understand some of those EMC lectures....not bad for a clanky ]

Ah, no. Grounding both ends makes a ground loop. Grounding only one end is preferred.

You can refer the below circuit, to know how they avoided the interference using a simple trick.

Here Cs is the standard capacitor of known value and we need to measure the current through the specimen. Both current will pass through different coils which wounded in a circular core as shown figure. Since the windings are in oppsite direction, the magnetic field created by them will be in opposite direction. The tertiary winding is placed in a way to catch the differential voltage due the differential magnetic field of both windings. So the signals due to the noice will get cancel each other and we will get the difference of standard capacitive current and the current through the specimen. Since the value of capacitor is known we can calculate the value of specimen current.

If your circuit is similar to this, it can use to avoid interference. But I don't know what is your circuit and whether it can be applicable for your circuit..