Shielded Cable for CT & PT Wiring
Shielded Cable for CT & PT Wiring
(OP)
Is it recommended to use shielded cable for CT and/or PT secondary wiring?
I'm working on a design for a small substation that will have MV cable and CT/PT/control wiring in the same concrete encased duct bank. Obviously the MV will be in separate conduits from the LV wiring. What's the minimum separation between MV and LV conduits you would be comfortable with and would you use shielded cable for the PT/CT secondaries?
I'm working on a design for a small substation that will have MV cable and CT/PT/control wiring in the same concrete encased duct bank. Obviously the MV will be in separate conduits from the LV wiring. What's the minimum separation between MV and LV conduits you would be comfortable with and would you use shielded cable for the PT/CT secondaries?






RE: Shielded Cable for CT & PT Wiring
Make sure your cable shields are only grounded at one end.
RE: Shielded Cable for CT & PT Wiring
Are you sure about only grounding one end of the shield though? The majority of what I've read so far recommended grounding both ends in this situation, at least for the CT wiring.
A typical 4-20mA signal cable should definitely only be grounded on one end, so it seems to me that we should only ground one end of the PT cables, but I don't believe grounding both ends of the CT cables would cause problems. But maybe it's a safer bet to just ground one end for the CTs too.
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
Ideally, the total length (from both ends of the cable) of the grounding bonding conductor for the cable shield should be less than 1% of the length of the cable run. This will give best result for transient suppression for the really high frequency noise you get from the arc of an opening disconnect for example.
See, for every bad bit of info I give, I'll have something good to use as well :)
RE: Shielded Cable for CT & PT Wiring
What's your opinion on the PT wiring? Seems like it might be wise to only ground one end since they should be carrying almost no current.
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
Yes, that was a mistype. Shield grounded on both ends, CT secondary only grounded at 1st point of use.
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
I have never used shielded cables for CT or PT circuits. However I have been limited to industrial (with limited utility) experience.
BTW, we are talking about (nominal) 5A CT and 120V PT circuits, right?
I am curious as to what type of LV cable is available in shielded construction. other than shielded instrument cables?
Historically I have used (Canadian) TECK type cables for CT & PT signals. For CT signals usually it's a #10awg conductor (or larger), and for PT signals it's usually #14awg or #12awg. TECK is similar to type MC.
TECK is available in shielded construction, but not till 5kV and #1awg in size.
Well, that's aboot all I have to say about that, eh!
RE: Shielded Cable for CT & PT Wiring
I'll start by saying that this is not for a utility substation, it's actually for a MV rotary UPS at an industrial facility. The wiring in question must run from the UPS SWGR enclosure to the UPS enclosure (SWGR and UPS will be located in two separate outdoor walk-in enclosures).
Yes, we're talking about typical 5A CTs and 120V PTs (although some of the CTs are 1A secondary).
Here's what I had in mind if using shielded cable:
http://www.houwire.com/hw152/
#10 4/C for the CTs & #12 4/C for the PTs.
Since I don't have access to IEEE 525 currently, and it's starting to seem to me that shielding may not be necessary (since so many places go without it), I'm leaning toward using un-shielded cable and increasing the spacing between MV conduits and control conduits in my duct. Opinions?
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
X2. To this day I never understood shielding of CT and PT cables. Perhaps if high accuracy is needed (guessing) but for typical protection class equipment I do not see a need.
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
The new control cables will be separated (CT's, controls, AC/DC), and the CT cables will be shielded at both ends. New utility practice, I guess.
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
RE: Shielded Cable for CT & PT Wiring
What about grounding a CT Circuit in more than 2 points? IE: 2 Marshalling Rack points and at the modules?
RE: Shielded Cable for CT & PT Wiring
Ground the CT cable shield at both ends (with proper techniques) for transient suppression. This is more important when running cables in a swithcyard where you will have exposure to a lot of different types of transient noise.
RE: Shielded Cable for CT & PT Wiring
I am a contractor commissioning engineer. Many of the stations in my area have changed ownership several times over the years. There is a new owner that is trying to upgrade the system, over time. In the case of the 27 conductor unshielded cable, we are changing out OCB's for GCB's and part of that is new shielded cables for CT's, with each CT set having its own dedicated cable & shield. The old cable was originally connected to EM relays + a SEL 221.
I've been in larger 345/138/25kV stations with similar 27 conductor cables and if a feeder breaker is open, the amount of induced signal is crazy / scary and can lead to all kinds of erroneous readings.
I do not disagree with what you have said, I was only pointing out that these things do exist in older stations.
RE: Shielded Cable for CT & PT Wiring
I completely agree, and would go as far to say that these problems exist way more in older stations. I believe partly because of brown field work over the years, perhaps being done to different standards. Partly to changes in work practices. Partly to the fact that the older EM relays were much more resilient to transients. So it's not necessarily that the problem wasn't always there, just that the consequence was not nearly as bad.
Most of my experience has been with older HV stations, which I prefer. They're more interesting work environments than the brand new installations. I've seen some EM relays get toasted, but not nearly on the scale that we've seen the newer IEDs get toasted. I've seen quite a few CVT boxes damaged as well. The particular CVTs we use (talking 230/500kV) are unusually high in capacitance, which can create even more problems with transients.
In my experience, proper grounding (and more importantly - proper grounding techniques) are not given the priority they deserve. I've seen teams of people chase gremlins in a system when the entire problem was a grounding conductor too long, or some cable was not properly shielded.