Originally Posted by goboatingnow
No that's ABYC s interpretation. It's not other code bodies interpretation. Your scenario can be simply protected by a combination of RCD on the secondary and outlet RCDs , providing the safety of both systems
From your side of the pond
Conflicting Philosophies for electrical safety
There is some controversy as to why ISO 13297 allows the use of RCDs as the sole means of protection against electric
shock without any other safeguards such as equipotential bonding (i.e. connecting AC protective earth to craft earth).
All other electrical safety standards expressly forbid the use of RCDs as a sole means of protection against electric
shock, and insist on equipotential bonding, especially in high risk areas.
Here is the relevant paragraph from EN ISO 13297 (which applies only to small pleasure craft in the EU area)
ISO 13297 section 4.2
The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as practicable to the battery (d.c.) negative terminal.
NOTE: If an RCD (whole-craft residual current device) or an isolating transformer is installed in the main supply circuit of the a.c. system (see Section 8.2), the negative ground terminal of the d.c. Systems need not be connected to the a.c. shore ground (protective conductor)
This particular part of ISO 13297 puts electrical safety totally in the hands of the RCD, but RCDs are not 100% reliable. Using an RCD alone without protective earthing is not acceptable on dry land or in a houseboat.
For example BS7671 section 415.1.2 states that;
The use of RCDs is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414
The only measure that can realistically be applied here is Section 411 which requires that fault protection is provided by protective earthing, protective equipotential bonding and automatic disconnection in case of a fault...
(BS 7671 411.1)
Note that BS 7671 is the UK version of the international standard EN 60364 electrical installation for buildings
which covers the EU area. It has sections applicable to marinas
and houseboats but is not applicable to small pleasure boats. However, the general philosophy of how to use mains electricity safely is consistent throughout the EU area;
a) use good quality earthing and Equipotential bonding.
b) an RCD provides additional
protection, particularly in high risk areas.
c) RCDs are not allowed as a sole
means of protection.
Seems inconsistent ? Why ?
The reason behind this earthing inconsistancy where EN ISO 13295 is different from all other advice is to reduce galvanic corrosion by preventing galvanic current flow from boat to shore through the (nonexistent) earth connection. However in the case where an RCD is the sole protection it does this at the expense of safety.
There is a perfectly simple way to have a safe protective earth connection without increasing corrosion and that is to connect the AC protective earth to the boat earth (battery negative, engine
block, any underwater metalwork etc.) and fit a galvanic isolator
. or an isolating transformer which will be safe and will not increase galvanic corrosion.
What are the consequences of a faulty RCD when there is no equipotential bonding ?
Let's say you are in the engine
room, perhaps doing some maintenance
work using an electric drill or a lamp powered from the AC mains.
Your boat is correctly wired in accordance with the electrical regulations
for small pleasure craft (ISO 13297 etc). In order to save money
on a galvanic isolator
or isolating transformer your AC mains earth and the boat's earth (engine block etc) are not connected together... this is OK according to the applicable regulations, but...
A fault develops in the drill or lamp and a live wire touches the engine block.
This type of fault should trip the RCD on the shore power
inlet, but the RCD has a fault of it's own and doesn't trip (see below for info about RCD reliability).
The engine block and anything connected to it is now live. That means the DC system is now live, the prop shaft and propeller
and sacrificial anodes are now live, and so the water
near the boat is also live.
around the boat will tend to conduct the live to earth but the connection is not reliable (especially on a plastic or wooden hulled boat) and is unlikely to be good enough to blow the main fuse or circuit breaker quickly enough (or at all) to protect anyone who happens to be in the water near the boat. Or a person on board who is touching the AC earth and the craft earth at the same time, maybe somebody is in the galley
touching the sink (boat earth) and the fridge (AC earth)...
This type of fault might kill immediately or it may go unnoticed for a long time, minutes, hours, days. Fire or electric shock are real possibilities and the longer the fault goes unnoticed the greater the chances of a serious outcome...
So what's the difference when equipotential bonding (earthing) is fitted ?
Go back to the previous scene in the engine room... if the AC earth was bonded to the boat earth (engine block etc) as soon as the live wire touches the engine block a fuse will blow (or the main circuit breaker in the shore power
outlet). This disconnects the power immediately. ... No danger
This has at least two valuable consequences, the first is that it immediately removes any dangerous voltage from the faulty item and therefore the engine block etc don't become live. The second is that it lets you know something is wrong... mmm drill doesn't work... why ?
Even if the RCD and
all the fuses
or circuit breakers were faulty and
the main earth connection at the shore power outlet is faulty as well, there is a much reduced danger when equipotential bonding is fitted because all the exposed metalwork and the water around the boat are at the same potential.
How reliable are RCDs ?
There have been at least three recent studies on RCD reliability
. The following is quoted from a 2007 report on RCD reliability
from the Electrical Safety Council in the UK. These studies were all done in domestic houses and all produced a failure rate of about 3 per hundred units tested.
The Electrical Safety Council's own survey
during 2006 of approx 600 RCDs in domestic properties showed a failure rate of 2.8 per hundred tested.
The Reliability of RCDs in Domestic Properties, ERA Report Number: 2007-0274, produced for the Electrical Safety Council 2007)
published in Italy
in 1996, electromechanical RCDs in Italian residential properties had an average failure rate of 7.1%. When the RCDs were subject to regular testing, the figure fell to 2.8%
From the available evidence, the primary mode of failure of the electromechanical RCDs tested in Italy
was ingress of fine particles of dust and moisture causing the moving components within the RCDs to stick or to operate more slowly than intended.
. Cantarella G., Caressin V., Tammasini R., Quality of Residual Current Operated
Circuit Breakers, ETEP. Vol. 6, No. 3, pp 149-156, 1996)
Electronic RCD manufacturers claim that their products are more reliable than electromechanical RCDs,. However, research
carried out in the US in 2001 suggests that the reliability of electronic RCDs may be similar to that of electromechanical RCDs, The RCD failures in the US were attributed to the failure of electronic components. The failure rates were higher in cities with conditions of high humidity, which may not be relevant to conditions found in the UK.
. GFCI Field Test Survey
Report, NEMA, Rosslyn, USA, Jan 2001)
Note that the above failure rates where in nice dry houses, increased failure rates are noted due to humidity and moisture ingress. There is no data for marinas
available however it seems reasonably to assume failure rates will be the same as or higher than in domestic properties on dry land.
The UK Electrical Safety Council report on RCD reliability is available here... RCD reliability