BHMA A156-28:2018 pdf download.STANDARD FOR RECOMMENDED PRACTICES FOR MECHANICAL KEYING SYSTEMS
Selective Master Keys
It is often useful in large keying systems to issue a high level master key to maintenance personnel which allows access across all master and grand master key boundaries of a keying system. A selective master key is usually very close to the system's top master key, both physically and through its realm of access.Nevertheless, it must be blocked from operating in many areas.
To designate that a lock be operated by a selective master key, suffix the symbol in parentheses to the standard symbol. Example: AA1 (ENG). This must be added every time the selective key is to operate and left off whenever the selective key is NOT to operate. For instance, you may lay out a system in which key AA1 is to operate two different offices. Inside one of them is an electrical cabinet which must be accessible to maintenance personnel carrying the ENG key. That lock must be specified as AA 1(ENG) while the lock for the other office must be specified as AA 1. Change keys directly under the selective key, such as ENG1,are operated by all higher level keys, such as GGM.
Like cross keying, a selective master key is a convenience feature which decreases the security of the cylinders it operates and limits the expansion and flexibility of the overall keying system. Each selective master key typically eliminates 20 - 25% of the capacity of the system where it is used, so it should only be specified when absolutely required. It is recommended that no more than one system-wide selective master key be used within the same system.
Cross Keving
Convenience May Reduce Cylinder Security and Hinder System Expansion
Whenever two or more different keys such as AA1 and AA2 are both required to operate the same cylinder,the cylinder's security is reduced. This is called cross keying. When the cross keying occurs under all the same higher level keys, such as AA1 and AA2, it is known as controlled cross keying. When it combines keys under different higher level keys, such as AA1 and AB1, it is known as uncontrolled cross keying.In addition to reducing the security of the cylinder, cross keying usually imposes limits on the flexibility and expansion of the overall keying system. This is especially true of uncontrolled cross keying.BHMA A156-28 pdf download.