advantage to having a board in the shed vs just sockets

throwaway657yf

i have a 20 amp b curve mcb in the board in the house protecting a 25 metre run of 2.5 swa in red ducting if revelent. in the shed i have 2 double sockets and on 1 2 way single gang light switch currently only one of the ways is being used for a 15 w led floodlight but i would like to add anothet floodlight possibly 50/75 watt. the load on the sockets is only 1kw from an pump and pressure vessel.

Would i be better off keeping the current set up or should i get it changed over to a dedicated sub board, with a rcd protecting witb 2 mcbs or is it worth the cost at all?

DublinDilbert

What are you planning on using the shed for in the future?

If everything is safe and working, i can't see advantage in changing it.

Traipsing through the rain to the house when the welder takes out the house MCB.
Tripping the entire premises on earth leakage instead of just locally and the walk of shame past the dwellers grumping their Z boxes and macinposhes just lost their data.


Sub-distro.

throwaway657yf

DublinDilbert wrote: »

What are you planning on using the shed for in the future?

If everything is safe and working, i can't see advantage in changing it.

atm its mainly a pump house, the occasional bit of grinding and power washing

Emperor.

An outbuilding requires isolation at the outbuilding, the main switch in a sub-board does that

Then there's the matter of RCD tripping locally at the outbuilding which is preferable to the house and the separate fusing of lighting and sockets

punisher5112

I'd definitely put it on its own rcbo out at the shed, I have to ask why SWA if it's in the red ESB suitable ducting????

There is always a risk of something going wrong out there for you to come home and find the fridge etc off due to the tripping.

throwaway657yf

punisher5112 wrote: »

I'd definitely put it on its own rcbo out at the shed, I have to ask why SWA if it's in the red ESB suitable ducting????

There is always a risk of something going wrong out there for you to come home and find the fridge etc off due to the tripping.

the next door neighbour is a farmer and occasionally drives his tractor on to my site when he is moving the cattle from one side of the road to the field on the other side

alan4cult

If the cable is isolated at the outbuilding what is protecting the cable between the two boards. Should there be two RCBOs, one at main board and one at sub board?

Currently I have seen setups with MCB at main board only and RCD at sub board.

Sartre.

alan4cult wrote: »

If the cable is isolated at the outbuilding what is protecting the cable between the two boards. Should there be two RCBOs, one at main board and one at sub board?

Currently I have seen setups with MCB at main board only and RCD at sub board.

Isolation at the outbuilding is to switch off the electricity at the outbuilding

The RCD/RCBO's are to protect the homeowner from electric shock

The RCD/RCBO's are fitted at the outbuilding to save the homeowner having to walk back and also means you can fit multiple RCBO's for lighting and sockets etc.at the outbuilding.

The SWA distribution cable is fully protected using an appropriate MCB or Fuse and doesn't need an RCBO.
Standard RCBO's shouldn't be cascaded either without time-delay, which isn't used much here if at all.
(Don't think there's any new rules on protecting SWA, would be unlikely anyhow)

zg3409

Is building insulated and heated?
If not it might get damp and that will cause issues for non waterproof sockets and fuse boards.

If you want to use a welder you might need a 16amp or 32 amp round blue socket as on a 13amp household plug they tend to blow the fuse.

As others have said if its damp or faulty the whole house might trip intermittently, and you won't be sure the fault is at the shed. My sky box intermittently loses power as the damp shed trios all the sockets in the house on very wet nights.

If going for DIY electrical I would not recommend a DIY fuseboard, just live with what you have and upgrade if required.

alan4cult

Sartre. wrote: »

Isolation at the outbuilding is to switch off the electricity at the outbuilding

The RCD/RCBO's are to protect the homeowner from electric shock

The RCD/RCBO's are fitted at the outbuilding to save the homeowner having to walk back and also means you can fit multiple RCBO's for lighting and sockets etc.at the outbuilding.

The SWA distribution cable is fully protected using an appropriate MCB or Fuse and doesn't need an RCBO.
Standard RCBO's shouldn't be cascaded either without time-delay, which isn't used much here if at all.
(Don't think there's any new rules on protecting SWA, would be unlikely anyhow)

Yes your last point is what surprises me. The SWA isn't protected from an RCD, so a shock hazard exists between the boards.
I'm guessing this same problem also exists between the meter and main board as the tails aren't RCD protected either relying on upstream protection from the supplier I guess.

keVo.

alan4cult wrote: »

Yes your last point is what surprises me. The SWA isn't protected from an RCD, so a shock hazard exists between the boards.
I'm guessing this same problem also exists between the meter and main board as the tails aren't RCD protected either relying on upstream protection from the supplier I guess.

The tails are protected by an MCB now in most cases

Not sure where you're getting the shock hazard idea from ,it doesn't really exist unless you cut through the pvc/pvc tails

Swa ,there's no shock hazard even if you cut it live, I've done it plenty of times

I was thinking SWA isn't protected against excavators either but usually people just draw the line under what's regulation.

I cascade RCDs; 60mA at the Master Distro and 30mA at the Sub. The knock-on benefit is when one fails the system still works and the overall probability of a defective device compromising the system is reduced from as high as 15% to a decimal.

keVo.

Sir Liamalot wrote: »

I was thinking SWA isn't protected against excavators either but usually people just draw the line under what's regulation.

I cascade RCDs; 60mA at the Master Distro and 30mA at the Sub. The knock-on benefit is when one fails the system still works and the overall probability of a defective device compromising the system is reduced from as high as 15% to a decimal.

Swa just trips a switch or blows fuses when you cut through it

You mightn't blow them all if you hit it and it's protected by fuses 3P

Not sure on that cascading 30ma and 60ma , discrimination usually requires time delay afaik

The wiring rules say not to do it afaik

keVo.

532.2.1.4

"Where RCDs are installed in series an RCD with time delay shall be chosen"

ET101 but new rules will have it as well

Yeah they often trip together, usually neither are incorrect in doing so. The fault current is simply in excess of 60mA. Time delay makes sense but spendy for high current jobbers.
Law of averages if only one trips it's the local one because of line impedance.

keVo.

Sir Liamalot wrote: »

Yeah they often trip together, usually neither are incorrect in doing so. The fault current is simply in excess of 60mA. Time delay makes sense but spendy for high current jobbers.
Law of averages if only one trips it's the local one because of line impedance.

It's considered 'bad practice' in electrical installations to have both trip together

The RCD downstream should clear the fault

Sure and do electricians call around to installations to test monthly and ensure that no trip under fault never happens?

keVo.

Sir Liamalot wrote: »

Sure and do electricians call around to installations to test monthly and ensure that no trip under fault never happens?

Homeowner supposed to test monthly, there should be a notice on the board

Not sure what last para means ?

I'm not a sparks so regs don't apply to me.

I agree

Best Practice
Time delay Master RCBO 60mA
Slave RCBO 30mA


Mediocre practice:
Master RCBO 60mA
Slave RCBO 30mA


Good Enough to Pass Regs:
RCBO 30mA local to the outbuilding.
Unbridled conductor set from the premises main input fuse to the outbuilding.

hesker

Sir Liamalot wrote: »

I'm not a sparks so regs don't apply to me.

I agree

Best Practice
Time delay Master RCBO 60mA
Slave RCBO 30mA


Mediocre practice:
Master RCBO 60mA
Slave RCBO 30mA


Good Enough to Pass Regs:
RCBO 30mA local to the outbuilding.
Unbridled conductor set from the premises main input fuse to the outbuilding.

Is this Master RCBO not subject to nuisance trips. Say you’re working on a socket circuit in your shed and the RCBO on that circuit is off. But your lights are on. If your socket circuit is single pole RCBO will you not get nuisance tripping of the Master as you break and make your neutrals

2011

Sir Liamalot wrote: »

Best Practice
Time delay Master RCBO 60mA
Slave RCBO 30mA

This is not something I would do and it would not be considered normal practice.

I would consider these to be the main considerations for a superior install:
- Fuse or MCB protecting shed supply cable (in main DB).
- Shed supply cable is buried and is SWA in a red duct.
- SWA contains a CPC i.e. does not rely on the armour only.
- SWA armour is earthed at both ends.
- Sub board in shed.
- RCBO for each circuit in the shed.

Let's not forget RCD's / RCBO's introduce the risk of nuisance tripping. Under some circumstances this can present considerable risk, for example when using power tools it wouldn't get great to loose the lights.
Therefore sometimes it may make sense to have more than one lighting circuit for a given area and avoid supplying the shed supply cable from an RCD.

Did I mention my shed is full of standby online back-up power supplies? :P
I tend to not be left in the dark.

I cascade RCDs all the time (plugging site distros into site distros) it works more often than it doesn't.
I can tick all the rest of the boxes on yer list except the red conduit is concrete slab and there's an RCBO without a time delay on the main distro.

I tend to use isolation transformers...separate gensets to be specific when I'm up to naughtiness that might involve "nuisance" tripping.

You can tell me there's no need for the series RCDs and I'll just keep saying I think it's safer, If I was determined I could trip it with an SDS and the system failsafe probability is far preferable.

hesker

hesker wrote: »

Is this Master RCBO not subject to nuisance trips. Say you’re working on a socket circuit in your shed and the RCBO on that circuit is off. But your lights are on. If your socket circuit is single pole RCBO will you not get nuisance tripping of the Master as you break and make your neutrals

That's a numpty trip not a nuisance trip...and yes it is subject to that but I'm not running a server room of SMPS power supplies with filthy noise filters dumping on the PE.
Neither trip very often usually it's because I'm being a thick.
A 14% failure rate of a single life saving device is not acceptable to me neither are unbridled cable runs with an input capable of igniting the cable jacket.

2011

Sir Liamalot wrote: »

You can tell me there's no need for the series RCDs and I'll just keep saying I think it's safer.

That depends entirely on the situation. Introducing more and more points of failure can in some situations introduce more risk.

The mitigation measures need to suit with the risk profile.

It is also important to appreciate that RCD's are only supposed to provide supplementary protection.

2011

Sir Liamalot wrote: »

I cascade RCDs; 60mA at the Master Distro and 30mA at the Sub. The knock-on benefit is when one fails the system still works and the overall probability of a defective device compromising the system is reduced from as high as 15% to a decimal.

I think there is something wrong with your maths!
Let's take the probability of failure of an RCD to be your valve of 15%.
If two RCD’s are wired in series the probability of both devices failing simultaneously is the probability of the first failing multiplied by the probability of the second failing.
Mathematically this is:

15% x 15% = 2.25%

So although an improvement it does not reduce the probability to "a decimal".

Furthermore, if the 30 mA RCD fails this leaves they associated circuit protected by a 60 mA RCD.
Although this is better than no RCD protection it must be recognised that 50 mA across the heart is considered to be potentially lethal.
Therefore it is important to note that in this example the remaining supplementary RCD protection should be considered suboptimal.

I think that there are better ways to reduce the probability of RCD failure such as regular testing (even if this is just pressing the test button).
Also spending an extra few bob on a high quality brand is likely to result in reduced failures.

I'm not a sparks so regs don't apply to me.

Sir Liamalot

Sir Liamalot wrote: »

I'm not a sparks so regs don't apply to me.

That was in response to bad practice considerations.


If I test monthly and use the 7% fail rate:

Bruthal wrote: »

Over a timeframe which gives a 7% failure rate, a failure of 2 in series at 7% each would be 0.49% failure rate for both within that timeframe.

But if 2 in series, we would never know one or both have become faulty without using the test buttons.

On the balance of practicality, 2 in series wouldnt be great.

2011

Sir Liamalot wrote: »

That was in response to bad practice considerations.

I know and I'm still confused

If I test monthly and use the 7% fail rate

Seriously???
That is a calculation using a completely different value!

You used 15%, not 7%.
Here is a direct quotation from this post of yours (bold added by me):

The knock-on benefit is when one fails the system still works and the overall probability of a defective device compromising the system is reduced from as high as 15% to a decimal.

^^^ Do you stand over this statement?? Yes/No

2011

2011 wrote: »

I know and I'm still confused

It means I don't do things just because I'm told, or it's regulation or that's the way we've always done it.
I make an assessment based on what I understand and choose the best course of action for me.
Regs. are advisory, I breach restricted works all the time (not for profit).
Sometimes I'm wrong, sometimes I'm right.
I've rectified plentya RECs work on my own property and non-grid connected installations to not be under the illusion that REC work is as significant as protagonists proclaim it to be.


At the end of the day which is better having a party with accountability or no incident? ...what I mean by this is having to go around and tighten terminals after a REC, or add an RCD to lighting circuits with metal switches, or insulate the chaffed cable in a metal pattress box etc...

2011 wrote: »

Seriously???
That is a calculation using a completely different value!

You used 15%, not 7%.
Here is a direct quotation from this post of yours (bold added by me):

That was a test! You passed..:o


15% is based on never pressing the test button, 7% is based on periodic testing. I remembered a prior conversation on the subject but not the finer details.

2011 wrote: »

^^^ Do you stand over this statement?? Yes/No

Not especially if we're splitting hairs; define failure, interpolate time frame, discriminate between brands etc..etc..


The fact of the matter is to paraphrase yourself I believe it's more likely I will drill through my SWA with an SDS than I am likely to saw my arm off with an angle grinder that's spinning down in a blackout that I have a prior provision to prevent from happening.


For my application 2 series earth leakage devices are less likely to not operate under a fault..I'll stand over that.



When I multiply the amount of consumer units in this country with a single earth leakage mitigation device present multiplied by the proclaimed failure rate....well golly 7% is hundreds of thousands.

2011

Sir Liamalot wrote: »

It means I don't do things just because I'm told, or it's regulation or that's the way we've always done it.

Do you consider all regulations optional or just some?

Regs. are advisory

Errr, no they most definitely are not.
I wouldn’t recommend testing that theory in court.
Have you any links to support this?

I breach restricted works all the time

Congratulations, but thats a statutory instrument not a regulation.
Many believe it was introduced to increase tax revenue, either way there is no technical element to it.

Sometimes I'm wrong, sometimes I'm right.

The problem is you are working with mains voltage which is potentially lethal.
Getting it wrong can have serious consequences.

I've rectified plentya RECs work on my own property and non-grid connected installations to not be under the illusion that REC work is as significant as protagonists proclaim it to be.

Off topic and does not justify ignoring regulations.

15% is based on never pressing the test button, 7% is based on periodic testing.

15% is also the value you came up with regardless of what it is based on.
You should consider a career in politics

Not especially if we're splitting hairs; define failure, interpolate time frame, discriminate between brands etc..etc..

I’m going to take “not especially” as a no. Fair play.

Anyway enough taking the pi55, I have found this exchange quite amusing

I have to admit I have been known for making similar errors.

2011

2011 wrote: »

Do you consider all regulations optional or just some?

All.

2011 wrote: »

Errr, no they most definitely are not.
I wouldn’t recommend testing that theory in court.
Have you any links to support this?

Disagree. Court is a manmade concept, along with laws and accountability. If we all stopped believing in them they'd cease to exist.

2011 wrote: »

Congratulations, but thats a statutory instrument not a regulation.
Many believe it was introduced to increase tax revenue, either way there is no technical element to it.

I found myself doing surgery on an overrated undersized relay contact today that was 16A rated and failed at 10A. It was so weedy the contact had bent like an MCB but not being bi-metal never bent back.

I bent it back. It's now "been compromised" and any certificate it had is invalid. The correct course of action would be to return it and wait a coupla weeks to have them issue me a repair or replacement that'll fail pretty soon instead.
Actually using a 16A relay in a 16A appliance in the manufacturing end is of course exempt of responsibility.

2011 wrote: »

The problem is you are working with mains voltage which is potentially lethal.
Getting it wrong can have serious consequences.

Yawn. I work on cars too. Physics are potentially lethal too.
Guess how many mechanics nearly killed me by forgetting to torque wheel studs?

Speed doesn't kill. Speed x mass kills.

Two RCDs = more lethal than one?!

2011 wrote: »

Off topic and does not justify ignoring regulations.

Is it? Supposedly RECs abide by regulations and I'm the guy who ends up earthing the switch plates.
I never said I ignore them I said they were advisory.

2011 wrote: »

15% is also the value you came up with regardless of what it is based on.
You should consider a career in politics

I'd never make it. I keep my promises and make good my commitments.
I'm a cr@p salesperson too.
"Waht..No it's shyte, I wouldn't be selling it if it was any good! What!? No I'm not reducing my price so...that's what they cost...I need all that money to buy two better ones"

2011 wrote: »

I’m going to take “not especially” as a no. Fair play.

There's an interesting characteristic amongst the electrotechnical fraternity; We like order, systematic approach, empirical evidence, defined and logical outcomes and arguing over the minorist detail with any booger who's miss-stepped...I do it as much as anyone.
We are woeful for holding people to account on minor inflections...

Ahem. "No. Fair play....with sugar on top"

2011 wrote: »

Anyway enough taking the pi55, I have found this exchange quite amusing

I know what you're up to....you're very good at it...and not alone..
Sometimes I learn stuff. Sometimes I appreciate another approach. Sometimes I just prod the bear.

2011

I see it like this:

1) You made a minor mathematical error but if you use someone else's numbers you get the desired answer
I agree that with your overall theory that 2 RCD's in series results in some level of risk reduction under some circumstances.

2) You say that all regulations are "advisory" but provide nothing to support this theory. My personal view is that you don't believe this, you just want to provoke a reaction. I agree that it is good to challenge regulations, especially the ones that I view as daft. I have been working with the HSA, believe me you are mistaken.

3) I also agree with you that a number of REC's blatantly ignore some of the rules and are responsible for some shoddy work practices. I don't think that this gives the rest of us a license to do whatever we like and I think that those REC's are a small minority.

4) You state that courts don't exist and you frequently break the law with respect to restricted works. I think this is your way of saying that you don't believe you will ever get caught, which is probably true.

5) Due to your experience, interest, resourcefulness and from tinkering with bits and bobs you feel that you have far more technical expertise that most / all electricians.

throwaway657yf

2011 wrote: »

I see it like this:

1) You made a minor mathematical error but if you use someone else's numbers you get the desired answer
I agree that with your overall theory that 2 RCD's in series results in some level of risk reduction under some circumstances.

2) You say that all regulations are "advisory" but provide nothing to support this theory. My personal view is that you don't believe this, you just want to provoke a reaction. I agree that it is good to challenge regulations, especially the ones that I view as daft. I have been working with the HSA, believe me you are mistaken.

3) I also agree with you that a number of REC's blatantly ignore some of the rules and are responsible for some shoddy work practices. I don't think that this gives the rest of us a license to do whatever we like and I think that those REC's are a small minority.

4) You state that courts don't exist and you frequently break the law with respect to restricted works. I think this is your way of saying that you don't believe you will ever get caught, which is probably true.

5) Due to your experience, interest, resourcefulness and from tinkering with bits and bobs you feel that you have far more technical expertise that most / all electricians.

i honestly didn't think this was such a topic for debate, i think ill call my electrician and ask directly about the sub board and get his opinion. on a kind of related issue if the mcb trips before the rcd is this due to overloading?

Yes. MCBs do overloading, RCDs guard unwanted paths or electrical leaks.

RCD = electrocution protection (life)
MCB = fire protection (cables)

Mikey..

Sir Liamalot wrote: »

Yes. MCBs do overloading, RCDs guard unwanted paths or electrical leaks.

RCD = electrocution protection (life)
MCB = fire protection (cables)

You seem to be self-taught in the electrical game

What happens ime is you get bits of information without getting the complete picture

Training and education in the specific field is needed to build the complete picture

I've done it myself where I researched an electrical area and then discovered I had made fundamental errors which were pointed out to me

2011

Mikey.. wrote: »

What happens ime is you get bits of information without getting the complete picture

To be fair I don’t think that applies in Sir Liam’s case

Never stop learning.

Thinking I know it all is the surest sign that I know very little.

Not only that but you never know until you know. I generally won't believe a property unless I've tested it myself and measured the results.