ZANDZ Technical Center has received a question that has resulted in this small article. We are sure that the answer to this question will promote making the right decision when designing lightning protection for critical structures.
"Please tell me how to determine if a lightning arrester is standalone (according to SO153.34.21-2003). The mast with the lightning arrester is distanced from the protected facility and its grounding device for not more than 4 m. I think that, in this situation, the path of the lightning current (or a part of the lightning current) will go through the grounding device of the protected facility, and such lightning arrester cannot be considered standalone. Is there a regulatory standard for this situation? Is my opinion correct?
According to definition provided in SO 153.34.21-2003, a standalone lightning arrester is a lightning arrester, the current collectors and the lightning rods of which are arranged such that the lightning current path would not be in contact with the protected facility. According to the same document (item 188.8.131.52), we cannot combine grounding device of the standalone lightning arrester with the grounding device of the facility.
But what if a part of the lightning current can flow through the protected facility or its grounding device, then such lightning arrester is considered a lightning arrester installed on the protected facility, according to SO 184.108.40.206-2003.
How can I understand whether the lightning current taken by the lightning arrester installed on the mast will flow across the grounding device of the facility (which is, e.g., located several meters from the mast)? And, respectively, should I combine the grounding device of the lightning arrester with the grounding device of the facility?"?”
The answer of the ZANDZ Technical Center:
Anton, the issue is somewhat more complex than SO 153.34.21-2003 states, but we can explain it in a simpler way.
I will talk about it point by point:
1. If we take into account RD 34.21.122-87, item 7 of Appendix 1:
"Standalone lightning arresters are those the supports of which are installed on the ground at some distance from the protected facility."
If we have to protect facilities according to Category 1, then the facility must be protected with a standalone lightning arrester. In this case, its grounding device should be also arranged separately (independently) and be spaced from the facility according to item 2.5 of RD 34.21.122-87.
If we protect facilities according to Category 2 or 3, then both standalone lightning arresters and lightning arresters installed on the protected facility can be used. In this case, standalone lightning arresters may have a grounding device combined with other grounding devices.
Anton, I tend to believe that this is the correct interpretation.
2. If we only consider SO, then your understanding is correct. But the definition of "the lightning arrester installed on a protected facility" seems incomplete.
3. Regarding the question marked in bold, then whether it is to combine grounding devices or not is a matter of facility classification according to the categories of RD 34.21.122-87 or according to the industry documents SO, STO, RD, etc.
4. There are documents that contradict the SO interpretation. For example, REVIEW OF GUIDELINES FOR OIL PLANT AND WAREHOUSE SAFETY. This is the primary document in this area since many other have been cancelled.
Item 78 states as follows:
"78. It is recommended to protect, against direct lightning strikes, tank farms containing highly flammable liquids and combustible liquids with total capacity of 100 thous. cubic m and more as well as tank farms of oil plants located in farmsteads with standalone lightning arresters."
While, at the same time:
"88. The grounding device to protect against the static electricity should be combined with the grounding devices of the electrical equipment and lightning protection. The allowed resistance of the grounding device intended solely for the protection against the static electricity should be not more than 100 Ohm."
Based on this, we can conclude that if we only use SO 153.34.21-2003, the standalone lightning arresters are those, which do not have either contact or a common grounding device with the protected facility. There are options available in all other cases.