Isolating Transformers, The Earth wire connection conundrum

[goboatingnow]

This discussion has surfaced from time to time , from 2008 onwards , especially as S/V Jedi has described his system.

The figure below gives my best understanding of what most would regard as best practice, and outlines the differences between EU boats to ISO 13297 and US boats to ABYC ( including the use of whole boat RCBs(ELCIs))

Firstly there is teh issue of nomenclature

'GND', 'Ground' , etc is a common return path for current , it may or may not be at earth potential

'Earth' , a point at earth or near earth potential

'Grounded'. usually synonymous with 'earthed '

'RCD" Residual Current Device , ( GFCI, ELCI etc)

'RCBO' an RCD and a overload breaker combined ( RCCB) This is UK terminology

'AC Protective Earth Wire' the common building wire at earth potential





This drawing represents an amalgam of US/EU practice. IN the EU, a shoreside RCBO is virtually always present certainly on marina pillars in recent memory.

The whole boat RCD ( usually RCBO in EU) is now effectively mandatory in both US and EU boats.

I have postulated best AC panel practice, which shows double pole circuit breakers on the AC feeds to the boat, optionally one could also have as is common in EU, a RCD, to provide protection for 'local' AC generators. equally a protective wire is run connected to the earth pins of outlets and teh cases of appliances in order to trigger breakers and RCDs if either side of the secondary touches these devices.

without an isolating transformer, we have two major differences

• DC negative is not commonly bonded to shore power earth wire
• Underwater metals are not bonded together and connected to the boat GND.

In teh US a deliberate AC seawater earth is sometime installed, but typically the bonding system and engine/prop shaft produce an effective GND to EARTH connection.


( one can see of course why US boats have huge issues with impressed corrosion, but thats an aside)

What I want to address here is the floating /non floating system as a result of installation of an isolating transformer

In ABYC solutions, where a local seawater earth is effectively established, the major safety factor of an isolating transformer, ie that earth is not in the fault path is removed. BUT, there is a robust debate, that isolating power transformers should not have floating secondaries. for example the common 110V tool transformer on UK building sites, is a centre tapped 110V secondary , with the centre tap connected to primary earth wire. Hence earth is in the fault return path but the max voltage that can be experienced in such a fault is 55V.

So we have a debate unpon installing an isolating transformer

1. A true floating AC output , no neutral, both are HOT, but earth is not in the return path, no danger to boat occupants if they touch one or the other HOT wires.

2. The inadvertent establishment of seawater(earth) referenced secondary output, in effect producing boat L & N . This could occur due to dampness, teh protective earth coming in contact with underwater or engine/prop shaft

3. The deliberate attempt to establish not only a local boat 'Ground", but to attempt to earth that ground, via underwater metals or even a hull plate.


Its clear that (3) is the preferred ( ABYC) US approach, yet such an approach adds earth as a fault path. ( note not a fault path TO earth). with this system if the secondary HOT ( L) touches anything on the boat and the user touches that, then a fault path to the neutral side of the transformer exists via seawater which causes a shock. ( this assumes the 'thing' is not connected to the protective wire).

What I dont understand is , by deliberately defeating the isolation it reintroduces the swimmer in freshwater risk, as fault currents can now flow through seawater , ie out of one underwater fiting and back through another, especially if the protective wire is poor made or connected.

ABYC, does not like (1) above and to be fair , several other code bodies do not like floating 1:1 POWER transformers. Why?

(a) a concern for excessive common mode voltages, but in practice the secondary does not float excessively above earth potential due to leakage resistance and capacitance .

(b) A rather strange concern ( as Calder as described) of the floating HOT touching an appliance, and raising the potential of that appliance unbeknownst to teh occupant, then a subsequent fault on the other HOT causes the occupant to be inserted into the circuit.

Now this is rather a bizarre claim, and seems to completely ignore two things
(a) A RCD on the secondary side covers this
(b) double pole breakers and the presence of the protective fault wire, will mean a trip will occur in most cases.

I have seen some fault sceanarios debated where tingling can be experienced on flating secondaries , because of stray capacitive connections causing low fault currents to return to the secondary 'neutral'

So an RCD and double pole CB protected secondary does deal with the secondary HOT to appliance ( or hull) issue, in that if the rather bizarre double fault occurs, then the RCD will sense the fault current returning via the protective wire. Thats assuming the CB hasn't tripped anyway.

SO while I can understand the theoretical ABYC concern, it does seem to effectively predate RCDs and proper protective (earth) wiring.

ABYC in effects , defeats a claimed safety feature of isolating transformers, to simply deal with the galvanic corrosion issue.

S/V Jedi on the other hand , went to a lot of bother to ensure that the safety aspect, ie no earth return path was maintained in his AC system.

some misc things

ISO 13297 offers no help by the way as it ignore isolating transformers completely

UK codes for liveaboards, requires equipotential earth throughout the boat, ie a polarising traffo is all that can be installed. earth must be continous from primary to secondary and out to shore.

Interesting in many European countries Neutral is floating, ie it is not at or near earth potential, Hence maintain L,N symmetry is not a priority and in fact the mains sockets are not polarised. ( resulting in lots of reverse polarity lights when a US boat plugs in , whereas reverse polarity isnt often detected on European boats). This obviously requires double pole circuit breakers to provide overcurrent protection in a floating neutral


Which in reality is right.

(ps Ive left the case earth connection issue for another day)

dave