Instrument Signal/Solenoid or Switch Cable Separation
Instrument Signal/Solenoid or Switch Cable Separation
(OP)
Customer specification calls for 12" separation for power and instrument signal cables running on cable tray.
Our designer did not consider 24VDC solenoid/switch power as a "power" cable (typically power assumed to be 220VAC for lighting etc) so we have analog and digital cable in same tray with a GRP barrier in the middle. Customer insists on 316sst barrier to prevent emf causing issue with signal cables.
Since 24VDC is low voltage and generates a small static magnetic field, will it be a problem for the 24VDC 4-20mA instrument signal?
The GRP barrier dielectric strength is 170 volts/mil. minimum and it's 3mm thick.
Any input appreciated.
Our designer did not consider 24VDC solenoid/switch power as a "power" cable (typically power assumed to be 220VAC for lighting etc) so we have analog and digital cable in same tray with a GRP barrier in the middle. Customer insists on 316sst barrier to prevent emf causing issue with signal cables.
Since 24VDC is low voltage and generates a small static magnetic field, will it be a problem for the 24VDC 4-20mA instrument signal?
The GRP barrier dielectric strength is 170 volts/mil. minimum and it's 3mm thick.
Any input appreciated.





RE: Instrument Signal/Solenoid or Switch Cable Separation
But even if there isn't a real EM problem, you might still have to meet the specs anyway.
RE: Instrument Signal/Solenoid or Switch Cable Separation
I'm not sure a metal tray barrier (the SST requirement probably is not relevant to EMI issue) is going to really solve the problem, if there is one.
If this doesn't involve miles and miles of circuits, you could propose using metal-clad cable for the 24V dc Solenoid circuits.
RE: Instrument Signal/Solenoid or Switch Cable Separation
But, as V1Bll says: You may have to follow the specs. Even if there is no technical reason for it. It all depends on what your relation to the customer (or inspector) is.
Gunnar Englund
www.gke.org
RE: Instrument Signal/Solenoid or Switch Cable Separation
RE: Instrument Signal/Solenoid or Switch Cable Separation
Gunnar Englund
www.gke.org
RE: Instrument Signal/Solenoid or Switch Cable Separation
We also used some which fit neatly onto terminal strips in the JB but some customers insist on being as close to the coil as possible.
RE: Instrument Signal/Solenoid or Switch Cable Separation
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Instrument Signal/Solenoid or Switch Cable Separation
Most of our solenoids are on SDV's which operate every two years depending on the testing frequency.
I just hope the customer will accept this solution.
RE: Instrument Signal/Solenoid or Switch Cable Separation
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Instrument Signal/Solenoid or Switch Cable Separation
I was waiting for that response. That myth (place diodes close to coil) has no physical or practical underpinning at all. I hear it very often. But it is wrong.
The switch-off transient (which is the worst interference source) occurs when the coil current is interrupted by a relay contact or switch (if the coil is switched via an SSR, the free-wheeling is already taken care of). Now, if you connect a diode at the driving end, the current doesnt get ripped off and there is no transient. What happens is that current decays slowly to zero. No high kick-back, no transients, no HF noise.
Gunnar Englund
www.gke.org
RE: Instrument Signal/Solenoid or Switch Cable Separation
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Instrument Signal/Solenoid or Switch Cable Separation
The magnetic field is very weak and causes no disturbance in a 4-20 mA circuit. It is the repeated refiring across the opening contacts that is the problem. It produces lots of HF interferece. Often into MHz. I shall try and find a recording I once made. That will convince you.
Gunnar Englund
www.gke.org
RE: Instrument Signal/Solenoid or Switch Cable Separation
Finaly, the contacts have separated so far - and the energy in the magnetic field has also been used up - that the final so-called Power Pulse is seen. It is the rounded wave-shape at the end of the transient.
The time scale is 20 microseconds/division and the vertical scale is 200 V/div.
Adding a diode parallel to the coil (or at the driving end) will give you a very boring 12 V DC level. No transients. No Power Pulse. No nothing. As I said before: Done that, been there. This is the way it works.
Gunnar Englund
www.gke.org
RE: Instrument Signal/Solenoid or Switch Cable Separation
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Instrument Signal/Solenoid or Switch Cable Separation
RE: Instrument Signal/Solenoid or Switch Cable Separation
Gunnar Englund
www.gke.org
RE: Instrument Signal/Solenoid or Switch Cable Separation
RE: Instrument Signal/Solenoid or Switch Cable Separation