Low voltage over top of high voltage

[raisinbran]

We have a situation where it would be advantageous to run a higher voltage power line (34.5 kV)underneath of a lower voltage (4.16 kV)power line. Both lines are on poles, but serve only our needs - mining. Assuming that we maintain adequate clearances, are there any rules/standards to prevent us from doing so? I have not seen the lines personally, so I do not have any additional information.
Regards,
Raisinbran

 

[wbd]

I have not seen a configuration like that but I haven't seen everything yet. Typically, it is the higher voltage is on top with the lower underneath, ie underbuild. The primary reason would be clearances. The higher voltage needs more clearance from ground level, therefore it is on top. For instance, say for the ground clearance over land, the clearance required for 34.5 kv is 45 ft and clearance is 30 feet for the 4.16 kv and 10 feet is required between the different voltages. With a underbuild, the pole could be 45 ft. With the lower voltage on top the pole would then be 55 ft. These numbers are just to illustrate my point and you would need to consult the NESC for specifics. Depending on your existing pole heights/class, you may be able to keep the poles by running spacer cable for each circuit.

 

[transmissiontowers]

The NESC is a public safety code to keep the general public safe from our overhead lines. If your line is entirely on your property and the public has no access, then you should be able to do what you want. As I understand the concept of higher over lower voltages, the idea is that the higher voltage should be more reliable than the lower voltage, so the lower voltage is placed lower on the pole where it might get hit by tall trucks or other obstacles. You probably should check with whatever governing authority is appropriate and see what they say.

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I have been called "A storehouse of worthless information" many times.

 

[wbd]

Even though if it is on your own property, if something happens, you may have some legal liability issues. This may be because there are accepted standards/codes in the electrical field and some lawyer may use that against you.

 

[waross]

The few lines that I have seen with two voltages were a transmission line over a distribution line. Another aspect was safer access to the distribution line for installation and servicing of distribution transformers. If your distribution system is the higher voltage and the lower voltage is a straight through feed with no loads connected in the area that the lines run together then it may be safer to put the lower voltage on top.
If the lower voltage line has any transformers then there may be serious access issues for your servicemen.

 

[BJC]

The "Normal" situation would be that the 4.16 Kv systee is downstream of the 34.5kV ( on the one line ). If that is the case you could take an outage on the 4.16 kV line for maintenance and not have to worry to much about the 34.5kV line. If the 4.16 line is on top and you need to work on it your going to have to shut down the 34.5 line as well.

 

[matwong]

Another reason i can think of is that higher voltage system can withstand higher impulse voltage especially from lightning. With the proposed construction, your equipment might be more susceptible to transient interference and moreover directed to your equipment which is undesirable.

 

[transmissiontowers]

Assuming that the top of the structure has a ground wire which will take the lightning strike, it does not matter much which voltage is on top. The back-flash from the lightning strike will flash over the lower voltage insulator. This is why a distribution under-build on a transmission pole/tower will flash over the shorter distribution insulators before it will flash over the much longer transmission insulators. (no insulator will stop a lightning strike)

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[tinfoil]

Question: are there many loads tapped onto the 4kv line? Overhead transformers and/or underground 4kV cable dips may have issues passing down through a 34.5kV underbuild.

 

[bacon4life]

The NESC Table 233 gives clearances for crossing circuits having the lower voltage on top, however footnote 9 cautions that "In general, this type of crossing is not recommended."

Table 235-5, which lists vertical clearance for conductors on the same structure, lists clearances for "Conductors and cables usually at upper levels" and "Conductors... usually at lower levels". The box corresponding to 4kV above 34.5kV is blank.

Seems clear this is not the preferred approach, but there may be some wiggle room for special applications. Perhaps you can use the 34.5 kV clearances for both circuits.

 

[mc5w]

There is an municipal electrical utility here in Ohio which places their distribution transformers OVER the primary wires which actually simplifies installing the transformer with a crane.

I have also seen an instance of service entrance running unbroken as aerial cable from a Duquesne Light Company pole to the top of a meter socket. This configuration was eventually outlawed in the early 1970s because stranded wire and service cables both act as poor excuses for water hoses.

Also, elastomeric potheads for single conductor shielded cable can be installed with the cable going into the top and the lug for open wiring on the bottom. Some electrical utilites do that for bringing power from an upper circuit past a lower circuit such as on dual circuit lines. The line fuses and transformers can then be below the lower circuit.

Rural Utilites Service does have a specification for mounting single phase transformers so that they are NOT directly underneath a single phase primary hot wire.

Every once in a while an electrical utility has a strange installation practice.