RCD for Isolating Transformer

[jddevel]

Finding I need to fit a RCD immediately adjacent to my Isolating Transformer which I hope/intend locating in a seating area in the rear deck space of my semi-trad what advice have forum members regarding a stand alone type weatherproof RCD being available for that purpose and also its size. 30mA or 100mA? Is it simply a case of landline mains to isolating transformer to top of RCD and from bottom of RCD positive to consumer unit and neutral bonded both to consumer unit and separate hull bonding or will neutral in consumer unit which is bonded to hull earth bond be sufficient. Hope I both understand the basics and have made myself clear.

[WotEver]

Why do you feel that you have to have an RCD 'immediately adjacent' to the IT?  Why can't it be in the CU?

[Robbo]

26 minutes ago, jddevel said:

Finding I need to fit a RCD immediately adjacent to my Isolating Transformer which I hope/intend locating in a seating area in the rear deck space of my semi-trad what advice have forum members regarding a stand alone type weatherproof RCD being available for that purpose and also its size. 30mA or 100mA? Is it simply a case of landline mains to isolating transformer to top of RCD and from bottom of RCD positive to consumer unit and neutral bonded both to consumer unit and separate hull bonding or will neutral in consumer unit which is bonded to hull earth bond be sufficient. Hope I both understand the basics and have made myself clear.

30mA,  How long cable between the IT and consumer unit?   It is basically Shore -> IT -> RCD -> MCB's/Neutral Bar.

[nicknorman]

I too can see no reason to have an RCD immediately after the IT. We just have one RCD on our boat, after the Combi and before any services. Bear in mind that except in exceptional circumstances, the shore supply will come from a bollard with an RCD in it, or even if it's a gash 13A socket, this will likely have an RCD behind it. There is no improvement in safety from having multiple RCDs in series. Neither BSS or Rec. Craft Dir. require it.

[Robbo]

8 minutes ago, nicknorman said:

I too can see no reason to have an RCD immediately after the IT. We just have one RCD on our boat, after the Combi and before any services. Bear in mind that except in exceptional circumstances, the shore supply will come from a bollard with an RCD in it, or even if it's a gash 13A socket, this will likely have an RCD behind it. There is no improvement in safety from having multiple RCDs in series. Neither BSS or Rec. Craft Dir. require it.

The IT is a separate source so the RCD on the bollard has no effect on anything downstream to the IT, so the onboard RCD should be as near to the IT as possible.  Mine is in flexible conduit from the IT to Combi then CU, if the cable is some length I would use metal flexible conduit for protection.

Edited August 24, 2017 by Robbo

[nicknorman]

8 minutes ago, Robbo said:

The IT is a separate source so the RCD on the bollard has no effect on anything downstream to the IT, so the RCD should be as near to the IT as possible.

Oh yes you are right on that first point! But anyway, you say "should be as near to the IT as possible" but where does it say that? Not in the BSS nor in the standards relating to the Rec. Craft Dir. The only requirement is to have an overcurrent device (which an RCD isn't) within a certain distance of the shore inlet (distance depending the nature of the cabling protection).

Unless the cabling between the IT and the consumer unit is very badly installed/protected, the chances of some fault that doesn't trip the shore MCB but does somehow electrocute a person, are minuscule. I'd be more worried about the little green aliens and taking extra care to ensure there were no gaps in my tin foil hat.

Edited August 24, 2017 by nicknorman

[Robbo]

Just now, nicknorman said:

Oh yes you are right on that first point! But anyway, you say "should be as near to the IT as possible" but where does it say that? Not in the BSS nor in the standards relating to the Rec. Craft Dir. The only requirement is to have an overcurrent device (which an RCD isn't) within a certain distance of the shore inlet (distance depending the nature of the cabling protection).

Unless the cabling between the IT and the consumer unit is bery badly installed/protected, the chances of some fault that doesn't trip the shore MCB but does somehow electrocute a person, are minuscule. I'd be more worried about the little green aliens and taking extra care to ensure there were no gaps in my tin foil hat.

Doesn't have to say in the BSS or standards to be good practise, and having a unprotected cable as short as possible and physically protected is sensible.

[nicknorman]

6 minutes ago, Robbo said:

Doesn't have to say in the BSS or standards to be good practise, and having a unprotected cable as short as possible and physically protected is sensible.

You could use armoured cable buried in a depleted uranium casing if you wish. But you will still die from something else!

It is poor engineering practice to spend money, add complexity and introduce an additional failure mode to address a risk that doesn't exist in reality.

Anyway, what is safer, a cable routed in trunking in the interstices of a steel boat, without an RCD before the CU, or that same cable brought out to be intentionally close to the occupants so it can be connected to an accessible RCD - the RCD being in a plastic case that won't tolerate eg being bashed with the anchor being deployed in a panic?

[jddevel]

3 hours ago, nicknorman said:

"should be as near to the IT as possible" but where does it say that? Not in the BSS nor in the standards relating to the Rec. Craft Dir. The only requirement is to have an overcurrent device (which an RCD isn't) within a certain distance of the shore inlet (distance depending the nature of the cabling protection).

Not wishing to start an argument I would say my point of reference as a complete novice with narrowboat electricity is in this particular case Tony Brooks` technical help website. Quote

Because the boat is bonded to earth (see metal boat earthing above) WITH AN RCD FITTED CLOSE TO THE ISOLATION TRANSFORMER you are protected from faults.

For me it`s belt and braces totally due to my ignorance.

3 hours ago, Robbo said:

Mine is in flexible conduit from the IT to Combi then CU,

Robbo are you saying you have a RCD " in flexible conduit"? Which I believe I may well be misunderstanding. If not exactly what sort of RCD have you got is it just in a small CU similar to that used for example in a domestic situation for an electric shower but with just the RCD in the housing?

[Robbo]

2 minutes ago, jddevel said:

Robbo are you saying you have a RCD " in flexible conduit"? Which I believe I may well be misunderstanding. If not exactly what sort of RCD have you got is it just in a small CU similar to that used for example in a domestic situation for an electric shower but with just the RCD in the housing?

No, the cable from the IT to the CU (which has the RCD) is in a flexible conduit.    My consumer unit is rather large as I have other stuff coming/going from it, but yes a RCD from the likes of of a shower CU.  30mA trip.  the loads amps isn't important as it is what the RCD can take not what it trips at.  Mine is something like 63amp.

Edited August 24, 2017 by Robbo

[jddevel]

4 hours ago, jddevel said:

or will neutral in consumer unit which is bonded to hull earth bond be sufficient

Thanks. Robbo so my original question which included the above quote is in fact O.K. The steel case forming the base of a seat will house the IT with a 2.5mm artic blue cable in flexible via the engine room to the consumer unit in the electric cupboard and the inlet will on the outside of this seat in a "tucked away" position.

[GUMPY]

As there has been sone nit picking on this thread here's some more:

Its very bad practice to have the IT on board the boat, it should be as near to the shore supply as possible. 

[Robbo]

Just now, Loddon said:

As there has been sone nit picking on this thread here's some more:

Its very bad practice to have the IT on board the boat, it should be as near to the shore supply as possible. 

Even more picky, IT with RCD if on shore.

[jddevel]

7 minutes ago, Loddon said:

Its very bad practice to have the IT on board the boat, it should be as near to the shore supply as possible. 

Now I`m confused! So am I to carry the IT onboard but when arriving at a marina unload it and carry it to the shore power location and then run my cable to the boat. Seems a bit odd but what do I know and is why I `m asking for advice. It`s difficult for the layman to often follow topics when there never seem a rigid consensus of opinion. Even more a concern when both safety and vessels health is concerned. According to Tony Brooks online advice - some quoted earlier - it appears that the neutral/earth bonding in an ideal and therefore best practice world should be attached from the RCD to the earth hull bonding. Who is right because I`m having real difficulty in following the various points which seem to indicate there is no certain way. Perhaps Tony will make comments at some point.

 

[WotEver]

5 hours ago, jddevel said:

will neutral in consumer unit which is bonded to hull earth bond be sufficient.

Seeing as you quoted yourself I'll take this opportunity to pick up on this. UNDER NO CIRCUMSTANCES should the neutral be bonded to earth from the CU. 

Edited August 24, 2017 by WotEver
Typo

[WotEver]

2 minutes ago, jddevel said:

Now I`m confused! So am I to carry the IT onboard but when arriving at a marina unload it and carry it to the shore power location and then run my cable to the boat. Seems a bit odd but what do I know and is why I `m asking for advice. It`s difficult for the layman to often follow topics when there never seem a rigid consensus of opinion. Even more a concern when both safety and vessels health is concerned. According to Tony Brooks online advice - some quoted earlier - it appears that the neutral/earth bonding in an ideal and therefore best practice world should be attached from the RCD to the earth hull bonding. Who is right because I`m having real difficulty in following the various points which seem to indicate there is no certain way. Perhaps Tony will make comments at some point.

Yes, there is a lot of confusion here. 

1) The IT should ideally be off the boat when in use. This is so that there is zero chance of any shore power conductor from directly contacting the boat. It doesn't need to be 'next to the bollard', that is just silly. 

2) Within the IT case the neutral & earth wires are connected together. This is the neutral-earth bond. 

3) Within the boat the cabling from the shore power connector should be inside trunking up to the Consumer Unit (or Combi if you have one of those). 

4) The CU should have a master RCD. The alternative would for the shore power inlet to go firstly to an RCD and then to the CU. The RCD in this instance can be adjacent to the CU. 

5) There should be a bond from the Earth busbar in the CU to the hull bonding point which should ideally be adjacent to but separate from the -ve bonding stud. 

[WotEver]

47 minutes ago, jddevel said:

Thanks. Robbo so my original question which included the above quote is in fact O.K. The steel case forming the base of a seat will house the IT with a 2.5mm artic blue cable in flexible via the engine room to the consumer unit in the electric cupboard and the inlet will on the outside of this seat in a "tucked away" position.

Ignoring the quoted bit (because it is wrong) the rest of the above would be fine* except that the arctic would be better in trunking and the RCD can be next to the CU. 

*Fine in the usually accepted meaning of the word - the sky won't start falling. Yes, the IT is better off the boat (in order to keep the shore power cable off the boat) but many folk have theirs fitted inside the boat for reasons of both convenience and security. 

[nicknorman]

1 hour ago, WotEver said:

Ignoring the quoted bit (because it is wrong) the rest of the above would be fine* except that the arctic would be better in trunking and the RCD can be next to the CU. 

*Fine in the usually accepted meaning of the word - the sky won't start falling. Yes, the IT is better off the boat (in order to keep the shore power cable off the boat) but many folk have theirs fitted inside the boat for reasons of both convenience and security. 

... and because you have to look at the big picture regarding safety. No point in going to extreme lengths to avoid an extremely remote chance of electrocution when as a result you trip over all the gubbins on the bank, bang your head and drown, or rupture your back picking up the IT and have to give up boating.

[cuthound]

2 hours ago, WotEver said:

Ignoring the quoted bit (because it is wrong) the rest of the above would be fine* except that the arctic would be better in trunking and the RCD can be next to the CU. 

*Fine in the usually accepted meaning of the word - the sky won't start falling. Yes, the IT is better off the boat (in order to keep the shore power cable off the boat) but many folk have theirs fitted inside the boat for reasons of both convenience and security. 

Another alternative would be to put the IT on the boat, but run the shore supply cable to it in plastic trunking or conduit, so that when the incoming cable is connected it could not short out to the hull if it's insulation became compromised.

[WotEver]

Just now, cuthound said:

Another alternative would be to put the IT on the boat, but run the shore supply cable to it in plastic trunking or conduit, so that when the incoming cable is connected it could not short out to the hull if it's insulation became compromised.

Absolutely

[cuthound]

1 minute ago, WotEver said:

Absolutely

It's how I used to solve the problem in telephone exchanoes where there are often more than one earth (several functional plus mains protective earths, but all connected together in the bottom of the building) to prevert "noise" on the protective earth from affecting sensitive equipment.

[GUMPY]

4 hours ago, WotEver said:

Yes, there is a lot of confusion here. 

1) The IT should ideally be off the boat when in use. This is so that there is zero chance of any shore power conductor from directly contacting the boat. It doesn't need to be 'next to the bollard', that is just silly. 

2) Within the IT case the neutral & earth wires are connected together. This is the neutral-earth bond.

I would be interested to hear your reasoning on point 1 as to why it doesn't matter where the IT is so long as its not on the boat;

on point 2 I think its wise to say that the N/E bond is on the secondary of the IT and there is no connection to the primary N or PE.

Today I have had a long conversation with a client trying to explain why a 3phase Delta to Y (or star if you prefer) transformer does not require a neutral on the input but has a neutral on the output!

 

Edited August 24, 2017 by Loddon

[WotEver]

23 minutes ago, Loddon said:

I would be interested to hear your reasoning on point 1 as to why it doesn't matter where the IT is so long as its not on the boat;

Think about its purpose. It's there to isolate the incoming earth. (It happens to also isolate incoming L & N but that's irrelevant to its purpose.)  Where it is located is irrelevant to that isolation. 

 

26 minutes ago, Loddon said:

on point 2 I think its wise to say that the N/E bond is on the secondary of the IT and there is no connection to the primary N or PE.

The OP was already confused with technicalities being brought into his thread and as the instructions with the IT would make the above perfectly clear I didn't see the need to add complication to a post the purpose of which was to simplify the issue for him. 

Hopefully if he is unsure about any wiring he will continue to ask questions as he has in this thread. 

30 minutes ago, Loddon said:

Today I have had a long conversation with a client trying to explain why a 3phase Delta to Y (or star if you prefer) transformer does not require a neutral on the input but has a neutral on the output!

I bet he still didn't get it.

[jddevel]

15 hours ago, cuthound said:

Another alternative would be to put the IT on the boat, but run the shore supply cable to it in plastic trunking or conduit, so that when the incoming cable is connected it could not short out to the hull if it's insulation became compromised.

[jddevel]

OK so that seems the logical way to go rather than lift a heavy IT of and on to the boat although not the perfect situation seems to reduce the percentages of danger to an acceptable level.

Plus all persons aboard will be obliged to wear a rubber overall with inbuilt lifejacket, gloves and duel purpose thick soled rubber wellingtons just in case.