Two firing heads can be combined at the top of a gun string to provide backup firing capability. For greater safety, both firing heads are positioned above the perforating guns and spaced apart from the guns with a safety spacer.

All individual safety features of the two redundant firing heads are preserved. No technical constraints dictate which firing head is activated first. Because the standard arrangement has an absolute pressure—activated firing head such as the HDF head as one of the systems, it is generally preferable to use this head as the primary firing head. If the primary head is a mechanical or electronic type and no shot is detected, pressuring the string to fire the secondary HDF could squeeze the formation of the primary head fired. The standard arrangement allows installation or removal of firing heads with the top shot below the rig floor.

The redundant fill sub configuration typically consists of the firing heads stacked one on top of the other in the same housing. The pressure-activated head is always installed on the bottom. If two pressure-activated heads are used, the primary head for firing should be installed on the bottom. Typical arrangements are BHF and HDF, TCF and HDF, and two HDF heads. Redundant systems also include the ProFire programmable firing head and eFire electronic firing head. Some specific applications require installation of the firing head after installing the completion equipment. In these situations, the preferred method is to install the HDF in TCF mode by well intervention methods (i.e., slickline or wireline).

Any head can be used as the upper head, but if a drop bar or TCF head assembly is used then the string must be fullbore to provide mechanical access to the upper firing head. The minimum ID of the string must be sufficiently large to allow dropping the drop bar to the BHF head or for TCF conveyance. The lower firing heads can be any of the eFire, HDF, ProFire, or DPF heads. The DPF head is used by combining the DFS with the fullbore redundant firing system.

Configurations of three firing heads are available when additional redundancy is important, such as in permanent completions. For example, two ProFire heads can be combined with a TCF head. The ProFire heads, run in place, are connected to a ballistic junction that propagates the detonation from the firing head simultaneously to X-Tools completion tools such as the MAXR or SXAR above and to the gun string below. The TCF is used only as necessary before pulling the completion.