6.3.3                 Electrical continuity for surface materials

6.3.3.1              Applicability

a.              The applicability of the electrical continuity requirements shall be established in accordance with the diagram in Figure 6‑1.

(a) Surfaces partially exposed include those exposed through small apertures such as venting holes.

(b) Multi-conductor structures exist where at least two conductive elements are separated by a dielectric surface of less than 100 cm2.

(c)  A capacitance of less than 10 pF is typically the case where the surface area is less than 1cm2. Keeping the capacitance low limits the size of any discharge. In LEO the short charging time as the spacecraft crosses the aurora limits the size of discharge even with a capacitor up to 100 pF.

(d)           Two dielectric patches within a 100 cm2 area are considered as one single dielectric for area or width estimates.

(e) This forms a triple point. For the definition of triple point, see 3.2.28. An example where this is important is the spot of non-conductive glue used to stick wires near the ends of an array string from the bare ends.

Figure 6‑1: Applicability of electrical continuity requirements

6.3.3.2              General

a.              All spacecraft surface materials shall be conductive to the extent that they lead to surface voltage below the maximum voltage specified in clause 6.2.1.

b.              If a surface material is used which is not conductive, to the extent specified in 6.3.3.2a, then it shall be tested to verify if hazardous ESD can occur or submitted to the customer.

c.               The determination of dc resistance of the surface material shall include the change over the service life of the bond in the environmental conditions which it is expected to experience including vacuum, temperature and mechanical stress.

6.3.3.3              Blankets

a.              All external and internal metallic layers of a thermal blanket shall be grounded to the structure with at least 2 bonding straps directly to ground (no daisy chain configuration).

b.              Any point on a blanket shall be within 1 m of a bonding strap.

c.               Adjacent blankets greater than 100 cm² in area shall be separately grounded to structure.

6.3.3.4              Material assemblies

a.              Material assemblies composed of different materials that are not separately grounded, shall be tested in their entirety.

NOTE              An example is optical solar reflectors.

b.              Metallic layers within material assemblies shall not be exposed to the plasma.

6.3.3.5              Conductive coatings

a.              The thickness of conductive coatings shall be such that there is no degradation of their performance as a conductor when exposed to the expected erosion due to sputtering and atomic oxygen.

6.3.3.6              Dielectric materials

a.              The following materials shall be avoided:

1.              Perforated metallized FEP or ETFE (SSM)

2.              Polyimide material with thickness above 50 µm if no ESD conductive coating is used on the space-exposed side

3.              FEP, ETFE, C2F4

4.              Epoxy glass

5.              Silica cloth

NOTE              The reason is that they have a known association with ESD risks.

b.              For dielectric materials, laid on top of a more conductive material, the resistivity of the dielectric shall be such that the electric field anywhere within the dielectric does not exceed EMAX =107 V/m.

NOTE              Normally, this is fulfilled through the requirement that the resistivity is less than the ratio of EMAX by the current density J.     
  
For a J of 10 nA cm-2,  W m.

c.               For a dielectric material, adjacent to a more conductive material and both exposed to vacuum, hereby constituting a triple point, the resistivity of the dielectric shall be such that the dielectric does not charge at any location to more than 1 kV negative or to more than 100 V positive with respect to the adjacent conductor.

NOTE 1      Normally, for a slab geometry, this is fulfilled through the requirement that the resistivity r is such that       

or

where t is the material thickness and d is the minimum distance at the surface of the dielectric to the adjacent grounded conductor.
or

where ρ is surface resistivity in Ω/square.

NOTE 2      The first equation in NOTE 1 is when the main conduction path is through the thickness of the material, e.g., to an underlying conductor while the second and third equations are when the main conduction path is parallel to the surface, e.g. to an adjacent conductor.
For a VMAX of 100 V and J of 10 nA cm-2,  W m2.               
For example, a 100 µm thickness dielectric must have a resistivity of less than 1010W.m if on top of a conductor and a resistivity of less than 106W.m if on top of a very resistive material and grounded every 1 cm.           
For a scientific mission where, say VMAX=10V,  W m2 usually implies that all surface materials must be good conductors.

NOTE 3      No margin is included in the values in NOTE 2.