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I am new to a group of engineers working on 13.8kV systems and do not want to appear stupid.

What are stress cones????  I assume that it is some sort of device that connects conductors to terminals on switchgear and transformers.  I also assume that they relief stress from the weight of a conductor and that they would supress corona.

What is BIL???  I assume that this stands for Basic Impulse Level.  Is this the level of voltage that a device can withstand without failing???

Thanks for the help and I enjoy this forum.  



Stress cones are required on shielded cables in medium voltage systems (2.4-15kV). In layman's terms (the limit of my understanding), when the cable is stripped to expose the shield, it must be stripped enough so that the shield can be pulled back away from the terminal by some distance determined by voltage level and stress cone type. I seem to think that this will be about 10-12" for a 15kV cable. The stress cone fits over the end of the cable and insulates the stripped portion as well as providing insulation between the terminal and the shield.  

I am not sure about BIL. I will see if I can come up something as I have seen that term before, just not sure where the reference is.


Stress cones are usually not required on 2.4/4 kv power cables in conduit, but are required on higher voltages.  Imagine concentric tubes of electrical field lines inside the cable, being confined by the outer semiconducting shield.  The entire cable from the center conductor to the semicon jacket then is full of these tubes.  When terminating the cable it is of course cut, then the semicon jacket is stripped back for a distance appropriate to the voltage.  If the cable is then energized you can imagine the tubes of the electrical field in the vicinity of the end of the semicon jacket will simply curl back around extremely sharply, while those near the center will curl back around at a greater arc.  The effect of this will be ultimate failure of the dielectric at the point where the semicon is terminated.  What you would have in effect is say 7200 volts separated from the grounded semicon jacket by only 1/4 inch of dielectric.  The solution to this is to install a "stress cone".  If you can now imagine a metal cone, say about 3 to 4 inches in diameter by 6 inches long, with the pointed end cut off, inserted onto the cable, small end first, to the end of the semicon jacket.  If the cable is now energized you can see that the tubes of the electrical field are curved in a much greater arc, so the electrical "stress is relieved".  In practice there are several ways to do this.  One is to use a manufactured "stress cone", which actually looks like a double ended cone.  One end is semicon, the other is dielectric.  The semicon is slid down so that it makes contact with the semicon jacket of the cable.  A taped up stress cone can be made by using insulating tape to make about a 3+ inch diameter ball at the end of the semicon jacket, then applying semicon tape to form the cone.  The semicon is made into a cone, and not allowed to curve back.  There is one method that uses some sort of chemical to make a stress relief that looks like a sieve, but I don't understand how that works.  The large porcelain potheads have stress relief built in.  

I think BIL is Basic Insulation Level refers to the ultimate voltage at which some device will flashover.  For example, an overhead primary distribution pole built for 12 kv will flashover at say, 60 kv.  Not sure what the proper voltage is, but that's the concept.


Excellent answer NormGa. thanks for taking the post past "layman's terms".


If I remember correctly BIL stands for Basic lightning impulse Insulation Level.  (three words go with the first letter).


BIL is basic lightning impulse insulation level.  Conventional BIL is the crest value of a standard lightning impulse for which the insulation shall not exhibit disruptive discharge when subjected to a specific number of applications of this impulse under specified conditions, applicable specifically to non-self-restoring insulations. (Definition from IEEE Std 1313.1, IEEE Standard for Insulation Coordination).

The usual test waveform for determining BIL of equipment is a 1.2 x 50 microsecond impulse, meaning the voltage pulse increases from zero to peak value in 1.2 microseconds and declines to one-half peak value in 50 microseconds.


Thanks so much for the info!!!
This place helps alot!!!



1. A Basic impulse Insulation Level that has been adopted as a preferred American National Standard voltage value is the preferred Basic impulse Insulation Level (insulation strength).  Reference ANSI Std C92.1-1971 "Voltage Values for Preferred Transient Insulation Levels"
Essentially, the insulation level is a combination of voltage values (both power frequency and impulse) that characterize the insulation of equipment with regard to its capacity of withstanding dielectric stresses
2. Type cones in advanced search of this Forum. It will return excellent postings on cable stress cones in addition to the starworthy postings above. Why not to use this Forum resources?


Just a point on BIL. I am working on an application, where all the conductors are inside a steel containment and not likely to be struck by lightning. However, the high voltage and currents used will develop some significant system ferro resonant energy. If a main power breaker suddenly opens, there will be a voltage spike as the system tries to maintain power flow. In fact, this is the principle that allows an automotive ignition coil to develop 30,000+ volts from 12 volts. The multiplication is not nearly as great here, but the components including the stress cones at the terminations must withstand the BIL level, or the system will quickly be damaged by switching transients.



This might be a little off the original post, but BIL discussion almost always leads me to surge protection for some reason.

If you find it difficult to achieve the BIL you need using only the stress cones, you also have the option of applying surge arresters to add to the protection provided by the BIL of the stress cones.  

As many of you have noted in this post, the voltage spike from some switching transients can be quite formidable.  Add the superposition of traveling waves at cable termination points, and you have some significant overvoltages.

IEEE C62.22.1-1996 is a good guide for applying surge arresters.


Suggestion to TheBlacksmith Aug. 3, 2001 posting:
The resonances and subharmonics that develop during a switchgear opening on its output side (if upstream is transformer winding) are usually suppressed by resitors to protect PTs for burning. The resonance or subharmonic resonance is caused by the transformer nonlinear inductive characteristics in conjunction with capacitive charge currents (or capacitive charges) of the downstream power distribution.


The purpose of the stress cone in the cable termination is to control the electric field distribution beyond the end of the semiconducting insulation . It minimize the field into the cable insulation and thus significantly reduces both the radial and longitudinal component of the stress at the end of the insulation shiled or the start of the stress cone as well as along the cable core interface
For high voltage application the stress cone is typically called in the USA Pothead. The material used are porcelain or polymer housing.

For specific info on stress cone visit the following sites:

Cable termination are cover in the IEEE Std 48-1990 "Test Procedures and Requirements for Alternating-Current Cable Terminations 2.5 kV Through 765 kV".   

The dielectric strength of cable termination are characterized by the typical standard test waves, power frequency voltage tests and Impulse tests (BIL and BSL). BSL is the Basic Switching Insulation level and consist of the standard 250/2500 microseconds.

BIL is applicable to all voltage ranges while BSL are significant above 230 kV per ANSI standard.

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