I understand that rules are rules, However I have never seen in many years of experience an inductive voltage problem with a 24 volt digital signal... For example thare are 10 1 hp 480vac motors with 10 photo electic sensors all in the same wireway all in seperate cables (non shieled). why would this be reason for a separation? If it were a 4 to 20ma signal sure...

My request is: Can anyone explain to me mathmatically or electrially that the inductance of a 3 conductor #10 cable at 460vac can cause or not cause a voltage spike suficient to trigger an AB PLC SLC 504 24vdc Digital input card at a distance of 30 feet? I would like to present the answer to the powers that be, however I am also a realist and don't think it will change matters... But I would like to know for sure....

 

Responding to Daniel Camacho's Apr 28 query:

Although you have never experienced a voltage induced problem I assume you accept that such (undesirable) effects can occur. Then, one must ask, rhetorically, what is unique about your installation?

First the qualitative analysis.
It can be shown that induced voltage is related to:

1) The mutual inductance between the power and control cable. Individual inductances can be neglected. Mutual inductance is inversely proportiona on separation distance.

2) Physical proximity of the two circuits.
The induced voltage is virtually non-existent if the cables cross at 90° and maximum if their circuit runs are parallel. Note, for the former I said "virtually" non-existent because even if the crossing is exactly 90° it depends on crossing symmetry.

3) Asymmetric power circuit current.
The current flow must be "seen" in the ground-return path common to the two circuits. Such current is nil in a three-phase power circuit because during normal operation the magnetic fields of the phase conductors are effectively cancelled. However, a ground-fault will produce an asymmetrical magnetic field sufficient to produce an induced voltage.

The same is true of of an asymmetrical phase current flow during motor starting. Of course, flow duration is crucial.

4) Construction / installation parameters.
Such factors tend to reduce induced voltage. Such factors include grounding, 3x1c cable, 1x3c cable, material of shield, if shielded. To a lesser extent, control cable shielding. In magor no-mag conduit. The effective ground-return-path size and routing.

5) The equation:

E = 2 x pi x f x M x I x L x K1 x K2, where

E = Induced voltage, Volts.
F = power system frequency, Hz.
M = Mutual inductance, uH / k-units.
I = Effective ground-current flow.
L = Circuit length, k-units.
K1 = Reduction factor.
K2 = Dimension factor, ft-yds-km.

In conclusion the fact that you have not experienced the effect(s) of induced voltage(s) is due to low normal operating current is low, earth-faults have not occurred, circuit length(s) is(are) short.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

two conductors separated by a non conductor is a cap. a cap will allow ac current to flow. it's conductivety is depending on the frequency. So is the energy transmitted. Running cables next to eachother a cap is born. Spikes do operate in high freq. aerea. gues high power spikes will be transmitted to the next cable pair. The impedance of that cable and so generated puls will produce a simulair spike form at yr 4-20 mA line. twisted pair will damp but not smother all. If your plc input is able to trigger of freq higher than the working freq of yr motor yr plc will trigger.Use an input filter (low pass f<line freq motor) to avoid this.

 

Why not put a scope on one of the 24 VDC inputs at the SLC, and do everything you can to cause transient conditions in the 460? Start motors, stop motors, start all of them simultaneously, etc. Set the scope to capture any glitches and see what happens. Then you'll have some quantitative feel for how big an issue it is for your particular installation.

BTW, are the 24V signals run in cable rated 460V or better? Since they're in the same raceway they are supposed to be. You could also change to shielded cable for the 24V signals, at least for the significant portion of the distance, if it makes you more comfortable.
That would be a lot cheaper than installing new dedicated wireway for the 24V.

--
Steve Myres, PE
Automation Solutions
(480) 813-1145

 

Your description is very unclear. You have stated that the motor conductors and photo-electric sensors are "all in the same wireway all in seperate cables". What is the separation that you are referring to? If the cables are all together, the cables are not separated. If you are questioning why the sensors have separate cables from each other, that would be difficult to answer without seeing the machine.

As to the second paragraph, the interference which a motor may cause to a 24VDC input is difficult to predict under all conditions. It is cheaper to be careful the first time, rather than doing it over a second time.

Separating the power and signal cables is also done because 300V insulation is more common on signal cables.

 

Depends on what you are doing with the motor leads. Turn off with a contactor could easily exceed the logic threshold of your input.
Harmonics from a VFD could also couple quite strongly.

Far too many variables to calculate whether it will happen or not. But, because people do things like that, most PLC inputs are very heavily filtered. Take a high bandwidth scope and look at your PLC input signals sometime. You'll come away amazed that the things work at all.

Regards
cww

 

Further to my response to Daniel Camacho's query:

Part Deux: the quantitative analysis.

It is far easier to measure the induced emf at either end of an open-ended 24 volt loop than it is to calculate.

On a more serious note, it is my opinion that the installation you described (admittedly detail is sparse) violates the NEC. Power and control circuits are permitted in the same raceway if derived from the same source of supply. An example would be power leads between a motor starter and its motor, plus its control leads derived from the same starter. Multi-motor runs are also approved if the motors work in concert, and are not normally operated independently.

Having been involved in several industries, wiring segregation was required practice, not for emf reduction, but to limit collateral damage resulting from electrical faults.

Of course, while the possibility of insulation failure in any circuit is possible the "risk" of such an event must be evaluated. My definition of risk is the resultant cost times the probability that it could happen. Unfortunately, while a great deal of attention is given the 3-phase fault, its probability of its occurrence is small. Conversely, the arcing ground-fault, while the most probable of all faults, seldom receives the design attention it warrants.

In closing, I'm sure those that experienced an arcing ground-fault agree that "putting all your eggs in the same raceway" is not wise!

Pro/con comments are strongly encouraged!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Further to my May 1, 12:55 response to Daniel Camacho's query:

L-units should have been mH/k-units.

I -units should have been kA.

Continuing Part Deux (sent after 12:55pm,) following are induced emf, neglecting input resistance of connected 24Vdc devices:

Case 1, Single Motor Operation.
The induced emf is about 15 mV.

Case 2, Electrical Ground-fault.
The induced emf is about 30 Volts (rms) for a far-end ground-fault current of about 4 kA (symmetrical). The corresponding asymmetrical first-cycle peak produces an emf of about 42 Volts. This case presumes there is no conductor damage due to burn-down or fusing in the raceway. It further presumes that: current-limiting fuses are not used; the raceway is metal; it is properly bonded to the ground-plane; the supply source capacity is about 300 kVA; the source is solidly-grounded.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])