In an Information Security Management System (ISMS), based on Recommendation ITU-T X.1051 and ISO/IEC standards 27001 and 27002, the physical security is a key issue. The electromagnetic interference caused by a High Power Electromagnetic (HPEM) attack and the ability to intercept information due to the unintentional electromagnetic emissions of equipment are significantly determined by the applied physical security measures. When the security is managed, it is necessary to evaluate the threat and mitigate either the equipment or site. The threat is related to "Vulnerability" and "Confidentiality" in ISMS. This Recommendation presents guidance on establishing the threat level presented by an intentional HPEM attack and the physical security measures that may be used to minimise this. The HPEM sources considered are those presented in IEC 61000-2-13 High-power electromagnetic (HPEM) environments - radiated and conducted as well as some additional sources that have emerged more recently. This Recommendation also gives vulnerability of equipment. The equipment is assumed to meet the immunity requirements presented within Recommendation ITU-T K.48 and relevant resistibility requirements, such as Recommendations ITU-T K.20, K.21 and K.45.