Effective Shielding of Cables
Because cables and lead wires can act as unintended antennas to both receive and to radiate electromagnetic interference (EMI) signal shielding is often necessary and is an important consideration of medical cable design.
Shields are additional conductors added to wire or cable to help isolate the electromagnetic fields of conductors within the shield from those outside of the shield. Shields may be placed over individual conductors, over groups of conductors, or over the entire bundle of conductors within the cable. It is often necessary to incorporate multiple shields, in which case, they are typically electrically isolated from each other.
The effectiveness of shielding is due to the shield reflecting a portion of the electro-magnetic interference but also absorbing a portion and directing it to ground. In both cases, some energy may still reach the signal conductors, but it is typically highly attenuated and does not cause problematic interference.
Physiological signals are analog by nature and are generally very low in signal strength. This makes any interference undesirable and therefore effective shielding of the signal may be an important consideration.
The most common location for a shield is immediately below the cable or wire outer jacket. However, more complex cables may be made up of several components (sub-cables) with one or more shields.
Shielding helps prevent unwanted external interference but is often used to prevent interference within the cable. This may be a significant factor when a cable has both power and signal lines, which is common for many medical applications.
Various shielding methods have advantages and disadvantages to be considered when selecting the most effective option for a particular medical cable application. In the next few posts, we will be reviewing spiral, braided, and foil shields.
Spiral or Serve Shields
A spiral shield, also known as a serve shield, consists of wire (usually tinned or silver plated copper) wrapped in one or more layers spirally around insulated conductors. Excellent flexibility and flex life as well as 85 to 95 percent coverage are characteristics of spiral shields. Spiral shields are also easy to terminate and are most effective at providing low frequency protection.
Spiral shields can lose effectiveness if the individual strands of the shield separate, something that can occur after a high number of flex cycles.
Next post: Braided shields