33kV and 415V Cable Separation

Should be in here someplace...

https://law.resource.org/pub/in/bis/S05/is.1255.1983.pdf

The ampacity of cable in channel and tunnel it is a very interesting issue, indeed. See-for instance:

From: Power Cables and Their Application, 3rd Edition Part 1 by Lothar Heinhold/Reimer Stubbe (Editors)[Siemens Akt.]

https://books.google.co.il/books/about/Power_cables_and_their_application.html?id=JOBSAAAAMAAJ&redir_esc=y

They are many other publications like these:

Power cables in tunnels by Birron M. Weedy

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4075071&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F10669%2F4075027%2F04075071.pdf%3Farnumber%3D4075071

TOWARDS A MORE FLEXIBLE RATING METHOD FOR CABLES IN TUNNELS

http://eprints.soton.ac.uk/267553/1/TOWARDS_A_MORE_FLEXIBLE_RATING_METHOD_FOR_CABLES_IN_TUNNELS_final.pdf

In my opinion, according to an approximate calculation-if no ventilation is provided and the ambient air-initial-it was 30oC a derating up to 75% of recommended in IEC 60364-5-52 [for low-voltage cable XLPE insulated in free air] and IEC 60502-2 for medium voltage cable.

What standard are you working to currently as it varies from standard to standard and country to country?

This is for a project in Malaysia. The client has some space constraints and so considering taking both LV and MV together. project adopts IEE and IEC. Thanks.

Thanks for your reply! I completely agree with you, but should not there be a way to find if the separation introduced here cancels out every possibilities of mutual induction even in the case of an fault in mv cable.

I asked many seniors and their comment was they could know it with experience.

Experience matters for sure. But how this practice started because I am not able to find any detail on separation of MV and LV cables that does not mention about physical barriers or to put it in different conduits.

The Client's standards, agreed in the contract, must prevail. However, it may be that this particular instance is not covered by the agreed standards, or it may be that an applicable standard has not been agreed, or it may be that a standard for this application does not exist. Therefore the best course of action is the technical discussion with the Client, as previously suggested, for commercial protection if nothing else.

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Given the separation I wouldn't have a problem with the proposal. But once one LV cable is installed the chances are that more will follow, that needs to be kept under control.

I agree with PWSlack.See what U.K. Gov. states:

Specification 034 Electrical Installations prepared by Defence Estates (DE) an agency of the Ministry of Defence [U.K.].

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/33549/spec34.pdf

TABLE 6A Section Six - Cable Installation MINIMUM SPACING OF CABLES

If we could consider the trough as cable tray:

NEC 2014 art. 392.20 Cable and Conductor Installation.

(B) Cables Rated Over 600 Volts. Cables rated over 600 volts and those rated 600 volts or less installed in the same cable tray shall comply with either of the following:

(1) The cables rated over 600 volts are Type MC.

(2) The cables rated over 600 volts are separated from the cables rated 600 volts or less by a solid fixed barrier of a material compatible with the cable tray.

As one could see, no distance is required if the medium voltage cable is MC type.

That standard while not stating minimum distance if Type MC rated cables are present, does absolutely mention a solid fixed barrier of compatible material. I read that to mean, no, do not put both (sets of) cables in that tray, do separate voltage classes by having essentially two completely separate trays.

You are right. Our practice is to segregate the cables as per

IEEE Std 525-1992, IEEE Guide for the Design and Installation of of Cable Systems in Substations

[I have not the 2007 edition with me. However, it seems to me the meaning of sentences remains the same]

4.2 Segregation. 4.2.1 High-voltage power cables

a) Installation of high-voltage cables in raceways that are separated from low-voltage power and control cables and from instrumentation cables. Installation of different voltage classes of high-voltage power cables in separate raceways is also suggested.

b) Utilization of armored shielded cables (separate raceways are still suggested).

However, separate raceways is not compulsory, in my opinion.

One has to test the high voltage cable shield and armor grounding often.

Thank you very much for the input and support friends.

I was so very unsure of this arrangement and I have proposed to my client to have a separate trough/trench for LV cable and not to take it along with MV cables.

I certainly did not understand why some of my colleagues and seniors told this arrangement is possible. But I preferred to have those cables separately, at least that is what IEE 525 says.

May be, as I was told by my seniors, after quite a while I would come to know things by experience and not by standards, but till then.........

Thank you all for the knowledge shared!

In my opinion, usually, the induced voltage, even in the case of short-circuit phase-to-earth or phase-to-ground-to-phase is low.

The distance-or separation-function it is to avoid fire damage from one to another-usually low voltage is less important, but medium voltage cable presents an elevated hazard of fire [the standard recommends the higher voltage in uppermost level since the fire gets up]. Once, the order was upside-down and I saw 6 stacks of cables bursting in a fire starting from a exploded junction box of a medium voltage cable located on the lowermost tray.

Yes, perhaps induced voltage is low, but can you imagine sensitive instrumentation wire openly in the same tunnel/channel with HV cable, and the HV system trip transient that results inducing a signal in the instrumentation wire that fries all that equipment?

If not fries it, then maybe it just messes up some seriously important data streams.

Actually it's a life and equipment safety issue, the goal being to prevent any connection between the high voltage circuit (via direct contact or during an arcing fault) and the low voltage circuit, plus isolating any ensuing fire spread.

Although equipment protection, signal integrity, overheating, and/or induced voltage, etc. issues are a consideration, those are design constraints usually covered in the engineering specifications documentation. Life safety is usually covered by independent standards; i.e., the AHJ doesn't care if the machine doesn't work properly, he just wants to be sure that nobody will die while using it (properly).