TE solutions provide reliable system protection against over-voltage and over-current conditions.


Replacing Trend with Wisdom

Is a 200kV BIL rating necessary for elbows protecting switchgear and transformer equipment? A correctly protected wind-power collector system enables effective and safe electrical distribution. By: Alan Tse, Director of Product Management, Energy

Over the past two years, TE has encountered renewable energy customers requiring 200 kV basic insulation level (BIL) rating on their 35 kV 600A deadfront elbows for switchgear and transformer equipment applications. The belief is that a 200 kV BIL rating provides greater system reliability and electrical safety. According to IEEE 386 Standard for Separable Insulated Connectors for Power Distribution Systems above 600V, the outlined BIL rating for a 34.5 kV system is 150 kV. From an overall system design and coordination perspective, the recommended use of 200 kV BIL is misleading and can compromise the equipment where being applied.

Reliable System Design

In system design, reliability is coordinated with system protection against over-voltage and over-current conditions. Note:

  • Surge arresters handle over-voltage conditions
  • Current interrupting products (fuses, reclosures, breakers, etc.) are designed to manage over-current conditions

When discussing BIL or basic insulation level, the concept relates to voltage withstand before insulation failure.  The voltage withstand of a system is designed so that the system can endure an impulse level up to the basic insulation level without permanently damaging equipment. System insulation level is not enough to provide sustainable reliability, which is the reason why typical electrical systems use surge arrester products and over insulated equipment. In transformer equipment, it has been common for some users to increase BIL through the primary windings. The increase in BIL margin can make a difference in enabling a transformer to survive longer. However, system insulation levels are not a guarantee to sustain reliability and for that reason typical electrical systems use surge arrester products.

Surge arresters in electrical systems have been in use since the early 1930s.  In the 1970s, underground surge arrester technology was developed for use in the increasing number of underground electrical systems. In an underground application, surge arresters are used to limit the amount of over-voltage that could be passed on to the equipment (e.g., pad-mounted transformers and switchgear). With today’s modern metal oxide arresters, when properly sized and working, the discharge voltage limits the amount of voltage seen across the equipment. Arresters protect the equipment. Note that the 200 kV BIL level of the T-body elbow has no significant value as it relates to over-voltage protection against switching surges or transient over-voltages.  

TE's elbows help wind contractors effectively distribute electricity.

The practice of over insulated equipment such as transformers has been adopted by some electrical utilities to provide improved system reliability when over-voltage protection on the system is no longer present.  When over-voltage protection such as an arrester is no longer functional, the next weakest point of the system must be considered to understand the mean time to repair (MTTR) and downtime revenue loss. In order to avoid costly damages it is common practice to over insulate the equipment in comparison to the easily replaceable components. As it relates to deadfront elbow terminations, it is recommended that the BIL rating of the elbow be less than the equipment, but equivalent to the electrical system. If the elbow termination BIL rating is equal to the equipment, the risk of expensive equipment failure increases. If you compare the MTTR of a 35 kV elbow to a transformer, the difference in time could be from one day to several months. In general, thermal loading will age and degrade the dielectric fluid and insulation material in a transformer. Please contact transformer equipment manufacturers to learn more about calculating loss of life.


If an arrester is operating properly to protect the electrical system, the likelihood of the equipment experiencing impulse voltages greater than 150 kV is minimal. The promotion of a complete 200 kV BIL system is not recommended. If the arrester is removed from the electrical system after reaching its end of life and all other components are rated 200 kV, the transformer can become the weak point, and become more susceptible to failure during the next over-voltage event. For greater system protection and safety, TE recommends using our 150 kV BIL elbows and mechanically connected metal oxide one-piece arrester. The TE elbow provides system reliability, easier install, and at a competitive price to provide higher level of safety to equipment and people.