General. Unless otherwise provided under paragraphs (b) and (c) of this section or ordered by the presiding officer under Rule 410(c), participants may obtain discovery of any matter, not privileged, that is relevant to the subject matter of the pending proceeding, including the existence, description, nature, custody, condition, and location of any books, documents, or other tangible things, and the identity and location of persons having any knowledge of any discoverable matter. It is not ground for objection that the information sought will be inadmissible in the Commission proceeding if the information sought appears reasonably calculated to lead to the discovery of admissible evidence.

Material prepared for litigation. A participant may not obtain discovery of material prepared in anticipation of litigation by another participant, unless that participant demonstrates a substantial need for the material and that substantially equivalent material cannot be obtained by other means without undue hardship. In ordering any such discovery, the presiding officer will prevent disclosure of the mental impressions, conclusions, opinions, or legal theories of an attorney.

Expert testimony. Unless otherwise restricted by the presiding officer under Rule 410(c), a participant may discover any facts known or opinions held by an expert concerning any relevant matters, not privileged. Such discovery will be permitted only if:

The expert is expected to be a witness at hearing; or

The expert is relied on by another expert who is expected to be a witness at hearing, and the participant seeking discovery shows a compelling need for the information and it cannot practicably be obtained by other means.