DEI Galvanic Isolators are typically sized to match the steady-state AC rating of the shore power service, which typically also provides an appropriate associated AC fault rating. If a higher AC fault current rating is needed, the following alternate methods may be used to select an isolator rating and model.

Alternate Product Selection Method - AC Fault Current

Exposure level: The grounding conductor of an AC voltage system is designed to carry the full phase-to-ground fault current in the event of a fault (e.g. shorted conductors or motor winding failures). Placed in this grounding conductor, the DEI device would be exposed to the entire available fault current, and should be rated for such.

Calculation method: The full available fault current from the shore power system is the value that we need to size for. The methods for sizing include (in order of preference):

a) using the breaker clearing curve for the circuit in question,
b) comparison of the grounding conductor size to fault withstand graphs for cables, or
c) calculation of the fault current at the shore transformer (worst case).

The three methods are described below:

a) Comparison to fuse/breaker clearing curves

If a fault occurs within the shore power cord up to the boat’s main breaker panel, then the shore breaker will clear the fault and ideally the shore breaker current/time trip ratings would be used. However, realistically sizing to the main breaker on the boat will adequately define the needed Galvanic Isolator fault rating. The breaker “total clearing time” curve should be used, and a DEI device rating should be chosen to exceed these curve values. On a clearing curve graph, the DEI device ratings should appear to the right of the breaker clearing curve. If you would like DEI assistance, please provide breaker clearing curves, or provide data so that we can find the curves in our library or on the web, including the breaker manufacturer’s name, model number, and ratings or settings.

b) Comparison of the grounding conductor to fault withstand graphs for cables

Device ratings can also be indirectly estimated by comparison to the AC fault capability of the conductor used on the circuit of interest. This is based on the assumption that the conductor was originally sized correctly for the available fault current, and will generally result in a conservative selection. Determine the grounding conductor size used in your shore power cord, then compare the current/time values in the withstand graph to the DEI device, choosing a product rating that approximates or exceeds the conductor rating. As this is an indirect method of fault sizing, it may be appropriate in some cases to have a DEI product rated below the conductor withstand value, if other factors, such as total conductor length, indicate that the fault current is substantially lower than the maximum conductor withstand.

c) Calculations based on the source fault current at the transformer

Unless the user has fault data for the exact circuit of interest, a conservative estimate can be achieved by starting with the fault current at the marina transformer. If desired, further refinement can be done by calculating current reductions due to conductor impedance due to the length of the service. This method is most useful for smaller electrical facilities; use at larger facilities may provide overly conservative values. Because the data needed will not likely be readily apparent, except to a site electrician, the other methods of sizing are recommended first.

The maximum fault current available at the transformer terminals is determined by the following formula, using data from the transformer nameplate, which can be supplied by a site engineer.

or

Secondary full load current =	Three-phase transformer kVA
	(Secondary kV)(√3)

then

Secondary fault current =	Secondary full load current x 100
	Percent transformer impedance

This is the maximum fault current at the transformer, which will be reduced by conductor impedance. This reduction can be estimated using wire characteristics for various cross-sections and lengths. Please contact Farwest for additional assistance.