technical article from Blue Sea Systems
September 17, 2007
Standard circuit protection may not guard against two ground fault hazards in marine AC circuits:
- AC electric power leakage from boats into the surrounding waters that may cause electrically induced drownings
- Low level ground faults that do not trip ordinary circuit breakers but can lead to fires and shocks to people on board
A ground fault occurs when current leaks from the AC hot wire to ground through faulty insulation, improper wiring, or failing appliances and devices. Ground Fault Circuit Interrupters (GFCI) found on most boats provide leakage current protection for equipment connected through a plug and receptacle. ABYC guidelines recommend 5mA GFCIs for receptacles in heads, galleys, machinery spaces, and weather decks.
However, wiring faults and shock exposure also can occur in permanent wiring and permanently connected appliances. Providing protection for these conditions requires additional ground fault protection.
Boat systems pose high risk. There is greater potential risk in electrical systems on boats compared to typical home or commercial wiring. The marine AC electrical system is supplied from shore by a power cord through plug and socket connections. The atmosphere is corrosive and the cord is subject to motion, chafing, and stress. Even though a safety ground conductor is supplied in the cord, it may not be completely reliable. A good electrical connection to the shore grounding system can fail in the marina wiring, the cord connections, or at connections inside the boat.
RCDs. GFCIs are Residual Current Devices (RCD). RCDs measure current flow in the hot and neutral wires. Current in these wires should be balanced-when 10 Amperes is flowing in the hot wire, 10 Amperes also should be flowing in the neutral wire (see diagram below). A current imbalance may be an indication of a ground fault-stray current leaking from the hot wire through defective insulation and possibly through a potential victim to ground. RCDs immediately switch electricity off when a ground fault is detected. GFCIs are RCDs with a trip level and timing that conform to a specific UL Standard.
RCBOs. Residual Current Breaker, Overload (RCBO) devices combine the ground fault protection of an RCD with the familiar overcurrent tripping characteristic of a normal circuit breaker. RCBOs can be installed as Branch circuit breakers providing 5mA ground fault protection, or as Main circuit breakers providing 30mA ground fault protection.
5mA vs. 30mA stray current protection. The sensitivity of GFCIs in the U.S. is 5mA to protect against shocks that might cause loss of muscle control for the most sensitive members of the population. 5mA sensitivity is ideal for single circuit protection; however, it is not suitable for whole system protection. The problem with 5mA sensitivity as a whole system solution is that typically there is cumulative leakage in excess of 5mA throughout a whole AC electrical system. Installing an RCBO with 5mA leakage trip sensitivity to protect an entire AC wiring network will result in nuisance tripping. Therefore, 5mA devices are restricted to use in single branch circuits.
In Europe, normal practice is to install a 30mA RCBO in the Main AC circuit as close to the power inlet as possible. This provides whole system protection. If a victim completes the circuit by touching a live conductive surface, normal AC line voltages will produce current in excess of 30mA through the typical human body. Therefore, 30mA protection can immediately switch the electricity off as quickly as a 5mA device in the event of direct human contact.
Protection from electrically-induced drowning. Increased interest in achieving a higher level of ground fault protection in the U.S. is generated by investigations into electrically induced drowning accidents in marinas. These investigations indicate that numerous drowning accidents in marinas have been caused by ground faults resulting in current leakage from boats into the surrounding water. An electrically induced drowning results from loss of muscle control caused by electrical current leaking into the water from boat fittings.
Electrical leakage into the water and the hazard it causes to swimmers is substantially different than touching a live wire on shore. Loss of muscle control can occur at 5mA. This might lead to the conclusion that for current leakage protection to be useful, it must be 5mA or lower. However, unlike a person on land touching a live wire and completing a fault circuit, water is conductive and will complete the fault circuit even if a swimmer is not present. In fact, when a fault develops, an RCBO that provides leakage protection of 30mA may break the circuit before a swimmer is in the area. Furthermore, when a swimmer is in the water, the swimmer shares leakage current with the surrounding water, so the swimmer's share of the leaking current will be less than the total current.
Individual circuit protection and whole boat protection. There is value in having both 30mA whole system protection and 5mA individual circuit protection. A 30mA Main circuit RCBO reduces fire and shock hazards from defects in permanently installed appliances such as water heaters, battery chargers, lighting fixtures, etc., and against defects in the wiring distribution system itself. A 5mA Branch circuit RCBO or GFCI provides a greater level of protection in specific areas such as heads, galleys, engine rooms, and weather decks where there is increased risk of shock. In boats with installed GFCIs, additional protection can be obtained by installing a Main circuit RCBO.
Additional information about RCBOs is available in the 2008 Blue Sea Systems catalog on pages 21.