I'm not sure exactly what type of armor you are referring to, but any metallic armor will provide significant shielding. It will not be as effective as individual conductor shields, and there will generally be portions of the circuit (near the terminations) where the armor must be cut off.
We generally run all instrumentation circuits in metal conduit for added shielding, even though the cable itself is already shielded.
Dear dpc,
the armor is a double tape aluminium or steel.I wanted to know whether we could earth both ends of armor and save on
buying a more expensive cable .
Grounding should only be done on one end as dpc pointed out. The armor will provide some shielding but not like that of regular signal wire. Signal wire (assume thats what your not going to use) is also twisted together. This cancels out the magnetic field produced by each conductor, providing noise immunity to common mode noise. If the wire you want to use is not twisted then you will have common mode noise that is conducted into the wires from the input side or the output side of this wire. This will not hurt the radiated noise immunity of the wire. Radiated is that noise that is in the air and penetrates electrical enclosures or wires. This is the noise, I believe, you are worried about. So bottom line is if the cable is twisted and you use armored cable then you should do quite well for noise immunity. Most of the armored cable I have seen, is wrapped around the cable. It does not provide a seal so there will still be some radiated noise getting into the cable. I hope this helps. Some more particulars about the cable would be helpful as well.
Dear Buzzp
the GIS manufacturer has said quote"The shielding is recommended although it is not absolutely necessary".
since the project is not a turn key project the manufacturer refuses to provide any information regarding
the level of radiation from GIS switching or the amount
that is transferred from earthing grid to the control
and protection panel.the main contractor offered to use
shielded cable .the cable is a multicore pvc sheathed cable
which has copper screen.I don't know whether this kind of
shield is what that we need.I thought may be the armore
that was already in order could be used instead.also I
read in the standard that a good shielding practice is the
continuity of the shield from GIS to the control panel.
so I thought that maybe we should earth both ends of the
screen.but it seems that I was wrong.
Yeah, I know only one end should be terminated. It should terminated at a point farthest away from the source (or maybe it is the closest?) Visit the following for a discussion of terminating resistors:
Is that picovolts, 144X? If so, you will definately require some shielding above and beyond the armor on the cable. I have never heard of communications using signals so small if this is the case. Maybe I don't understand p.u? Thanks Buzzp
Dear Buzzp,
No ,it's not picovolts
p.u voltage means (actual voltage)/(base voltage) which in the example I told you it means about 2*132KV(for a 132 KV system) at the CVT input for example which is transferred
to the control cubicle through the secondary side of the CVT
the frequency is very high but duration is very short.
and there is no communication problem here only disturbance
to the protection equipment.
Oh okay. Sorry for the ignorance. Is the protection equipment nuisance tripping on some particular fault or does it just reset? You may consider putting some ferrite cores around the protection equipment conductors that are suspected of conducting in the noise. This will help some for sure. Without seeing an input waveform into the protection relay it is hard to select the proper ferrite core. I am sure there is some significant voltage or current into the protection relay so a ferrite core with the proper ratings may be hard to find. What inputs and outputs are there on the protection equipment that is causing problems? I was thinking filter but at those voltages that will be a nightmare. Have you been able to capture a waveform into the protection equipment to verify noise is an issue? One other question: Are the control wires running with wires carrying significant current? This is a big no-no if this is the case. I am not trying to insult your intelligence just covering all the basics. Thanks, Buzzp
we have no experience on this subject.just worried about
the hint that manufacturer gave us.and the current in the
control and protection cables are not high.all of the relays
are numeric with a a very low burden.
I forgot to tell you somthing.the CT and VT outputs are
going first to a local control cubicle through shielded
and coaxial cables and then they go to the local control and protection panels and we are worried about this route.
by the way there still remains the problem of noise through
ground.
I was just saying not to run control wires with motor leads, for example. Do I understand that there are erratic problems with the control? Or there is no real problem yet just a concern? It sounds like the GIS is suspect here. I could be more help if you want to send me a drawing of your system and identify the wire type as well as what other wires are ran with the wires. Also, identify what type of waveforms are on each one (DC, AC with freq range, and maybe amplitudes).
