Shielded Cable for CT & PT Wiring 2

[Fritzy92]

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?

 

[marks1080]

absolutely!

Make sure your cable shields are only grounded at one end.

 

[Fritzy92]

Thanks for the response!

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.

 

[marks1080]

I'm sorry Fritzy, you are correct. Must of had a brain fart, not sure why I would write that. Shield grounded at both ends.

 

[marks1080]

Make sure the length of your bonding conductors connecting the cable shield to ground are as short as possible. Grounding via a cable gland will give you the best result.

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

 

[Fritzy92]

Marks, thanks for all the info. Understood, we'll ground the CT wiring on both ends and keep the bonding conductors as short as possible.

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.

 

[marks1080]

I ground the PT cable shields the same way as for CTs. This should keep the voltage profile at each end of the cable shield the same.

 

[stevenal]

we'll ground the CT wiring on both ends...

Mistyped perhaps? The shield is grounded at both ends, the secondary wiring is grounded at one location; usually at the 1st point of use.

 

[Fritzy92]

Mistyped perhaps? The shield is grounded at both ends, the secondary wiring is grounded at one location; usually at the 1st point of use.

Yes, that was a mistype. Shield grounded on both ends, CT secondary only grounded at 1st point of use.

 

[bacon4life]

My utility does not use shielded cable for CT or PT wiring in medium voltage or 115 kV substations, consistent with IEEE 525. If you do use shielded cable, ground both ends of the sheild. In order to avoid damaging the shield from excess current flow, you must run a large ground wire in close proximity to the shielded cable as explained in IEEE 525 Section G.

 

[marks1080]

Good addition bacon! Minimizing the geometric area between your grounded cable shield and the ground grid will lead to better transient suppression.

 

[GroovyGuy]

Hi Fritzy92,
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!

 

[Fritzy92]

Thanks for all the responses, definitely some good information!

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?

 

[slavag]

We are always ground shield of any control cable ( include CT, VT) on the both side.

 

[Mbrooke]

My utility does not use shielded cable for CT or PT wiring in medium voltage or 115 kV substations, consistent with IEEE 525. If you do use shielded cable, ground both ends of the sheild. In order to avoid damaging the shield from excess current flow, you must run a large ground wire in close proximity to the shielded cable as explained in IEEE 525 Section G.

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.

 

[GroovyGuy]

I agree with Mbrooke; shielding of these CT & PT cables is not required. Even more so since (the OP) indicates that the MV conductors will be installed in conduit. Since the MV conductors are surely to be shielded, and I'll assume some form of metallic conduit, you will not have any issues with noise. As far as spacing is concerned, I would think a couple of feet would be prudent, assuming that you are not running parallel for hundreds of feet.

 

[DTR2011]

For what it is worth, I am changing out 2 138kV Oil Circuit Breakers soon. The existing cabling to the breakers is a 27 conductor unshielded cable. This includes CT's and all other controls/status from the breaker. Sub is about 40 years old (replacing E/M distance /BF/Reclosing relays as well). The distance from the control house to the breakers is 260'. So, unshielded cables seemed to work fine for many years.

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.

 

[Mbrooke]

Can you ask them why? Thats strange imo.

 

[marks1080]

DTR2011 I think it's a good idea to seperate the analog signals from the controls. You may have gotten away with that set up with electro-mech relaying, but if you're moving to IEDs you would likely be dealing with a lot of mis-ops.

 

[Sn00ze]

Might be going off on a tangent here but I am curious. What's the advantage of grounding on both sides? wouldn't that create circulating currents via inductance?

What about grounding a CT Circuit in more than 2 points? IE: 2 Marshalling Rack points and at the modules?