Triboelectric Noise in Medical Cables and Wires
Noise can come from many sources external to the cable but also from the cable or wire itself. Noise generated within a cable is often called handling noise or cable noise, but this type of unwanted signal is more accurately described as triboelectric noise.When measuring low-level signals, noise in cable or wire may be problematic. Noise in an ECG or other medical signal may make accurate diagnosis difficult or even impossible.
Recognizing that amount of triboelectric noise in medical cables should be minimized, The AAMI/ANSI EC53 requires the maximum peak-to-peak noise shall be less than 50 micro-volts (µV). Many device manufacturers specify even lower noise limits for their cables and leadwires.
What is Triboelectric Noise?
The triboelectric effect is a phenomenon in which an electrical charge is generated by the friction between dissimilar materials which commonly occurs when they are rubbed together. The amount of charge generated is largely dependent on the composition of the materials and the amount of friction between the materials. Within medical cable assemblies and leadwires, random triboelectric noise is generated when the various conductors, insulation, and fillers rub against each other as the cable is flexed during movement.
Keeping triboelectric noise at acceptable levels requires careful material selection, design and processing as cable material is manufactured.
Testing for Triboelectric Noise
Testing for triboelectric noise is done on cable or wire material, not cable assemblies. EC53 section 5.5.4 specifies to “test a representative sample of cable material….” A common question is why finished cables are not tested. Besides the directive to test cable material, 7’ of cable is needed for the test, and most cables assemblies do not have that long of an uninterrupted span of cable material. More significantly, movement at any termination point within the connector or cable assembly will typically generate a much greater amount of artifact than the noise generated by the triboelectric effect.
Test Set-Up and Test Procedure
ANSI/AAMI details the test setup in section 5.5.4 and in Figure 8 of the EC53 standard. A compliant set-up will include 36” high, heavy gauge steel posts been into a concrete or other sturdy floor set five feet apart, center-to-center. A one-half inch thick steel plate is centered on the top of each post allowing 5’ of cable or wire to be held firmly between clamps set 48” apart. Care must be taken in locating the test area away from interference from electrical panels, large electrical motors, vehicle traffic or foot traffic.
A weight equal to 40 times the weight of 1 foot of cable or wire is attached at the center of the wire to be tested. The weight held level to the cable clamps and is dropped while voltage measurements are taken using a digital oscilloscope.
Designing to Reduce Triboelectric Noise
A design process that begins with establishing and documenting performance and regulatory requirements is important in the development of medical cable assemblies and the cable material that will be used as part of the assembly.
Within multi-conductor cable the greater the number of conductors and surrounding material, the greater the opportunity to generate triboelectric noise. Selecting materials that slip on each other easily increases flexibility; however, care must be taken to select materials that when rubbed together do not produce unacceptable levels of noise.
Cable noise can also be reduced by adding conductive materials and elements within the cable or wire that drain or dissipate the triboelectric charge away from the wire insulation.
Most diagnostic devices incorporate noise filtration or compensation; however, reducing noise at the source typically improves diagnostic quality. Triboelectric noise can be reduced to very low levels by incorporating low-noise cable and wire material and designing connectors and strain reliefs to prevent any movement at the termination of conductors to the contacts.
Let the Affinity engineering team assist with your projects that require low noise cable assemblies or leadwires. We have the knowledge and experience to help make your interconnect project successful.
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