On 2005-02-26 12:38:34 +1100, Sir Spamalot said:

On Fri, 25 Feb 2005 22:27:31 GMT, John Proctor
wrote:

On 2005-02-25 12:44:20 +1100, chuck said:

Hello Eric,

Thanks for clarifying.

I believe you are correct. You are talking about high-resistance
onboard leakages that generate currents too small to be detected by the
GFI circuit or the breakers. The isolator diodes would probably not
conduct under those circumstances and a capacitor would help.

UL requires the GFI to trip at a 5 ma unbalance, so 24,000 ohms of
leakage would trip it. Actually, the isolator diodes would probably
pass 5 ma in that circuit without a capacitor. The capacitor would be
necessary when the leakage resistance was in the megohms and the
currents in the microamps.

Would rather not have that stuff flowing through my ground connections
through the water to adjacent boats, even at those low current levels.
This underscores the importance of making sure you don't have dangerous
leakages onboard in the first place. Easy enough to check, but how many
regularly test their GFIs?

We sure agree on the isolation transformer, too.

Thanks again, Eric.

Chuck

Chuck,

I am puzzled by your UL 5 ma rating.

Here in Australia we have basically two ratings. The first is for most
domestic installations an is 30 ma. The second is for hazerdous
locations including hospitals where the rating is 10 ma. There are
other higher rating RCD (Residual Current Devices) which are used in
industrial contexts (eg 60 ma) but the majority of installations are
either 30 ma. or 10 ma.

BTW I am an EE as well as a licensed Electrical Contractor.


Chuck,

Now **I** am puzzled by the Australian ratings, especially since my
sister lives "across the pond" in NZ. 30ma has been known to kill,
hence I question the 30ma "domestic installation" rating. And you
guys are on 240V, correct?

Are we discussing semantics here? In the US, a GFI (120V) is supposed
to protect the unknowing public from faults that could channel more
than 5ma through the body. UL/CSA/others test to that 5ma
specification.

SS


PS: Me too on the EE business.

Curiouser and curiouser...

In Australia we have generally what is called a multiply earthed
neutral system (MEN). There are other types of distributution systems
used in remote areas but MEN system is found in all urban areas. In
this system n+1 service conductors (where n = number of phases) are
brought into the consumer's premises although if available on the
distributiuon system 3 phase power is much easier to obtain here than
in NA (I originally came from Canada). This phase active and netral in
a single phase supply is the normal residential. The neutral is boded
to earth at each consumer switchboard and the earth is bonded to a
ground system which is normally a grounding rod of reasonable size.
Some allowances are made for installations where a ground rod is not
practical or where it cannot be of sufficient size. It has not been
acceptable to ground to utility services (gas or water) for some number
of years due to lack of guaranteed continuity (gas reticulation is via
plastic pipes and water meters har generally non metalic internally).

I have generally seen in the literature that 60 ma is generally
considered the level at which statistically fibrillation is almost
guaranteed when the current passes through the chest cavity. Some of
the references given seem to indicate much lower levels. However when
standards are set they must take into account the statistical nature of
some of the effects they are trying to deal with. Hence I think the 30
ma residential rating here in Australia. The AS/NZS 3000:2000 Wiring
Rules specifies an maximum RCD rating of 30 ma for residential circuits
and I don't believe there are any addendums which mandate a lower
maximum. As I indicated in my previous post the 10 ma level is required
in hospitals or other high risk application areas. However you need to
balance emi filtering requirements with leakage levels so that you
don't get nuisance tripping.

I don't know about New Zealand but given the nature of trans-Tasman
standards activities I would be surprised if their wiring rules were
significantly different from ours. After all we use the same wiring
rules!

In residential installations an RCD (residual current device) is
required on general purpose outlets and lighting circuits but does not
need to be applied to direct connection appliances like cooktops or
stoves. However practice has evolved so that 'safety switches' (RCDs)
are placed on the whole installation (for older installations) as these
are easier to wire into old and cramped switchboards. New installations
segregate the power, lighting and direct connected appliances and have
an RCD on the appropriate circuits. Some contractors also will provide
unprotected circuits for essential circuits (refrigerators, freezers
etc.) to prevent food spoilage from nuisance tripping where the outlet
is set up for fixed use.

In the case of marinas there is usually a 16A branch circuit protected
by a 30 ma RCD supplying shore power to the vessel. Unfortuanately, I
don't have any experience with larger vessels where 3 phase power from
shore is delivered but I would expect the situaltion to be similar with
n+1+1 conductors being delivered to the boat (where n = number of
phases and the extra +1 is the earth). It is mandatory that earth and
neutral be kept separate on the vessel. Installations where there are
changeover facilities to a genset require the earth and neutral to be
bonded only when the genset is the power source.

Sorry about the length but that is generally how things work downunder.

--
Regards,
John Proctor VK3JP, VKV6789
S/V Chagall