Currently I only have RCD protected circuits in my house (One for sockets & fixed appliances, and one for the lighting (Tripping one does not affect the other)
(I am planning on re arranging this, putting the lighting, freezers and alarm system on non RCD circuits)
Currently the shed feed is from a 20A MCB connected to the "sockets" RCD (30mA) via a 4mm SWA Cable. This cable is about 25m, however the shed is only about 4m from the house. The house earth has been exported to the shed.
From the shed CU, via another 20A MCB (Connected to the shed 30mA RCD) another 4mm SWA cable then feeds a smaller CU in the garage with yet another
30mA RCD (Only three MCB's) This cable is about 30m. (The garages are right up the top of the garden, as far away from the house as can be!)
The garage is then earthed with an earth rod. (It is about 35m from the house) (The SWA cable's armour is connected to the earth in the shed, but isolated in the garage)
My questions are... Should the garage feed simply piggyback the feed TO the shed CU, eliminating the RCD and MCB there?
Should the feed from the house to the shed be via a 100mA RCD, separate to the rest of the house?
I assume the earth in the garage should be isolated from the earth in the house/shed, as it's earthing arrangements are different? (House earth is provided from the electricity company (TN-S) where as the garage has it's on rod (TT)
Another question is, why do I need to TT the supply in the garage? Can't I just use the house earth? (My logic is, the earth provided by the electricity company is at least 500m long (That's where the substation is!)
(Patio is currently being re laid, so if I need to do any rewiring, now is the time)
Another question is, why do I need to TT the supply in the garage? Can'
I just use the house earth? (My logic is, the earth provided by the electricity company is at leas
500m long (That's where the substation is!)
(Patio is currently being re laid, so if I need to do any rewiring, no is the time)
Sparks... The simple answer to this question is that unless you are an approve electrical contractor under part "P" you can't do this as sheds an garages are classed as special risk areas. If you wish to do it yoursel you must be able to show competence and notify the local buildin inspectorate before carrying out the work and they will appoint a inspector / tester to verify the work before it is re -energised. Thi will cost the same as getting an approved electrician to do the job o even more than
On the other hand the cheapest, legal and therefore one has to assume the most safest method, is to run an extension cable to the shed from a socket. This is apparently called progress.
No need then to lie in bed worrying that the IEE or Part P police will
come knocking on your door at 5am demanding to conduct an inspection.
-- There are no Part P police, thats a bit silly to say the least. What does matter is that if you have reason to claim on your insuranc and the work is not certified the Insurance company will just wal away, even if the claim is not related to the electrical work. th other thing is that after September ths year , the sale of a house will contain a statement from the current owner that any work carrie out complies with the building regs. finally from January 2005 th house owner is legally responsible for the electrical safety of th property. I dont say the new law is good, in fact it has nothing to d with safety but a lot to do with tax collestion. I th real aim wa safety they would have made it illegal for anyone but a qualifie electrician to do any work, but that might lose votes
What you have is basically OK; all the rambling that follows is in the realm of minor tweaks towards unattainable perfection. That said... the cascade of RCDs and MCBs is not best practice - should you have a fault in the garage, you could well end up with not only the garage power being cut (which we want) but also the shed (pain) and the house sockets (double-pain). What you want to get to is to have the cables to shed and garage protected against short-circuit and overcurrent, while leaving protection of the loads they feed to a dedicated RCD and closer-rated MCB in each place (shed and garage). To achieve this, you want to feed the 'submain' - the supply to shed and garage - from sthg like a 20A
*fuse* (not MCB: you actively *want* the slower reaction to overloads of a fuse here) on the non-RCD side of the house CU. That should feed into the little shed CU, and 'daisy-chain' (i.e. not passing through any way of the shed CU) on to the garage. In both shed and garage your small CU has its own RCD (or if you're often working at night, separate RCBOs for lights and power in the garage, so that earth leakage fault from power tool cable-slicing oopsie ;-) doesn't plunge you into darkness).
That's the outline of the 'best' solution. However, to install it and be sure you've met required disconnection times for the supply cable will need Real final-circuit design calculations - they're not hard, but do require reference to tabulated values of cable characteristics.
As to earthing - again, what you have sounds about right (shed close to house shares its earth, more distant garage has its own rod). If your supply is PME (TN-C-S) there's an argument for making even the shed a TT installation, but it's not an overwhelming one. The reason PMEness matters here is that (in handwavy outline) PME creates one bigass 'equipotential zone' (kinda like supplementary bonding does in one bathroom, on the whole-house scale), so we stop caring about 'true' earth potential; and with metallic incoming services bonded to, the installation earth is going to be v. close to 'true' earth anyway. Once you're 30m away, like your garage is, and given that a single fault in your buried feed could lose the bonding between N and E, it makes increasingly little sense to rely on the (you-hope-it's-been-)exported house earth; so making the garage its own little TT installation - as you already have - is the Right Answer.
It'd certainly be better for it not to share the house sockets' 30mA RCD
- doing that increases the probability of nuisance trips in the house (both from full-on faults Outside, and just from outside appliances adding a few mA of their legitimate leakage to 'preload' this shared RCD close to its tripping point). As I whittered above, it's OK for the supplies themselves not to have any RCD protection, provided the socket circuits they feed do end up with RCD protection. If you feel happier running through a 100mA RCD, you can, but you can't rely on that necessarily discriminating with the garage RCD. (The strength of that argument again differs with what you're doing in the garage: if you've one light in there and one socket where you plug in a vac to clean the car, a single whole-garage RCD is fine; if it's a uk.d-i-y.hardcore garage, you've got no room for a car, but a couple of benches, two pillar drills (one of which works and the other's going to Real Soon Now), a boatload of other power tools, some unreasonably dangerous electroplating lashup in the corner, oh and the feed to the combination Thunderbirds launchpit-and-burglar-pit, and so on - in which case you'd want the whole garage TT to have its own 100mA time-delay RCD, and either a splitload 30mA-RCD CU or individual RCBOs for (at minimum) the socket and fixed-equipment circs... In that case, you want the house end of things to protect *only* the feed cable, leaving all the RCDing local to the garage.
Hope I've outlined the reasoning above. For TN-S, the exporting issue is less sharp than for TN-C-S, but for a 'remote' garage 30m away you're increasing the earth loop impedance a fair bit from what the supplier's big-ass incomer gave you as you pass it down your wimpy little 4mmsq SWA to the end of the garden. And with PN-C-S becoming more widespread, even as a 'retrofit' as suppliers meet increased local demand from all that good brownfield development, it makes sense to make new/upgraded installations 'PME-aware' ;-)
Well, the garage is basically for the car (at the moment!) The shed is my "work room" for fixing PC's (My job) so no real risk of me wielding a huge angle grinder in there, then getting plunged into darkness! I have installed an emergency light in the shed anyway, as I had one going spare ;-)
Is there any value in having the house earth in the garage as well, and also connecting this to the rod? - or is this a definite no?
I have been planning on adding a second, smaller CU to the house, with just an isolator and a few MCB's for the freezer and alarm system, I think I will just add the outside stuff to this new CU (On a 32A Type1 MCB)
Fair point, makes sense to future proof these things as much as possible, as there is already a rod, I wont take it away!
Thanks for taking to trouble to explain this, I did think it was basically there, just needed a few minor adjustments!
I have just done some calculations for the house -> shed run... using
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to Shed Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved Basic cable properties: Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth conductor Installation method: clipped to a surface Uncorrected nominal current: 36 amps at 30 degrees celcius Rated full load temperature: 70 degrees celcius Ambient temperature: 30 degrees celcius Current rating temperature correction factor: 1 Semi-enclosed fuse correction factor: 1 Grouping method: no grouping; cables widely separated Cables in group: 1 Grouping correction factor: 1 Ring correction factor: 1 Length in thermal insulation: 0 Thermal insulation correction factor: 1 Corrected nominal current: 36 amps Specific resistance of power conductor: 0.00461 ohms/metre at 20 degrees celcius Specific resistance of earth conductor: 0.0121 ohms/metre at 20 degrees celcius Temperature cofficient of resistance: 0.004 K-1 Specific resistance of power conductor: 0.005532 ohms/metre at 70 degrees celcius Specific resistance of earth conductor: 0.01452 ohms/metre at 70 degrees celcius
Voltage drop: For a total cable length of: 25 metres Volt drop at end of cable: 8.8512 V when carrying 32 amps and cable temperature is at maximum value of 70 degrees celcius Basic compatibility of overcurrent device with cable and design current Selected over-current device: 32-amp type-1 MCB Nominal current rating of overcurrent device is compatible with design current Nominal current rating of overcurrent device is compatible with corrected cable current rating Current required to disconnect device in 5 seconds: 128 amps Effective worst-case earth conductor resistance of cable: 0.363 ohms at maximum rated temperature Worst-case shock voltage after five seconds: 46.464 volts Disconnection times Total fault resistance: 1.3013 ohms Fault current: 176.746 amps Over-current device will disconnect in 5 seconds Over-current device will disconnect in less than 0.4 seconds This cable/device combination is suitable for all allowed applications, including bathroom and outdoor systems
Now from the shed to the garage (Assuming it's on a 20A MCB in the shed):-
Shed to Garage Generated by CableCalc V1.0 (c)2000 Kevin Boone, all rights reserved Basic cable properties: Cable type: PVC-insulated two-core-and-earth 4 mm2 with 1.5 mm2 earth conductor Installation method: clipped to a surface Uncorrected nominal current: 36 amps at 30 degrees celcius Rated full load temperature: 70 degrees celcius Ambient temperature: 30 degrees celcius Current rating temperature correction factor: 1 Semi-enclosed fuse correction factor: 1 Grouping method: no grouping; cables widely separated Cables in group: 1 Grouping correction factor: 1 Ring correction factor: 1 Length in thermal insulation: 0 Thermal insulation correction factor: 1 Corrected nominal current: 36 amps Specific resistance of power conductor: 0.00461 ohms/metre at 20 degrees celcius Specific resistance of earth conductor: 0.0121 ohms/metre at 20 degrees celcius Temperature cofficient of resistance: 0.004 K-1 Specific resistance of power conductor: 0.005532 ohms/metre at 70 degrees celcius Specific resistance of earth conductor: 0.01452 ohms/metre at 70 degrees celcius
Voltage drop: For a total cable length of: 30 metres Volt drop at end of cable: 6.6384 V when carrying 20 amps and cable temperature is at maximum value of 70 degrees celcius Basic compatibility of overcurrent device with cable and design current Selected over-current device: 32-amp type-1 MCB Nominal current rating of overcurrent device is compatible with design current Nominal current rating of overcurrent device is compatible with corrected cable current rating Current required to disconnect device in 5 seconds: 128 amps Effective worst-case earth conductor resistance of cable: 0.4356 ohms at maximum rated temperature Worst-case shock voltage after five seconds: 55.7568 volts Disconnection times Total fault resistance: 1.40156 ohms Fault current: 164.103 amps Over-current device will disconnect in 5 seconds Over-current device will disconnect in less than 0.4 seconds This cable/device combination is suitable for all allowed applications, including bathroom and outdoor systems
All looks OK to me, and that would be exporting the earth to the garage via
A definite no. I believe you wrote you had the SWA armour connected to the house earth at the house end, but isolated from the garage CU - that's just what you want to make the garage a pukka TT system. If you did connect the house earth to the garage rod, under fault conditions (not necessarily in your own house, but for example another place close by on the same phase of the same substation) you might end up with quite substantial fault currents coursing through your earth wiring; not really desirable...
Liar, liar, pants on fire ;-) You said in your original post it was SWA. If 3-core (L,N,E all get an inner core of their own, armour earthed Because It Should Be rather than to carry fault currents) then the earth conductor's the same size - 4mmsq - as the L & N. If it's 2-core with the sheath being used as the E conductor, you need to use the right Table (or hope that CableCalc has access to its contents) to use the impedance of the armour.
That's the second time you've said that ;-) I take it, then, that the CU is more or less in the opposite corner of the house to the shed, so that although the shed's only 4m from the nearest bit of house, the total SWA run is 25m as you say...
So at (unrealistic) full load, that's just about your entire voltage drop budget eaten in this first 25m (conventionally we allow up to 4% as cable loss - for the 240V supply that's 9.6V). Unlike the earth impedance calc, this isn't affected by SWA-versus-T&E, since the live conductors are the same size (4mmsq).
Then you take another 25m (and you lie again about the cable construction ;-) for the segment from shed on to garage. But the calcs from CableCalc believe this is a 25m segment from the origin of the supply; so you need to (a) add the voltage drop for this segment to the already-established 8.8V drop for the first run - putting you over the voltage drop limit for drawing a heftier load in the garage; and (b) establish clearly what the earth fault path actually is for an L-to-E-conductor fault right at the end of the garage run. If that path relies on the house CU breaker, you need to factor in the full 50m of the combined run to do your fault-path calcs (which I now notice you mention, at the end of your message, do assume the reduced-cross-section protective conductor of T&E, while you know you have 4mmsq:).
Though we're not actually 'exporting' the house earth to the garage, are we? (The garage end is TT). But the house earth is what an L-to-E fault at the end of that supply cable will go back through (and you've got the worst-case 0.8ohm earth impedance of your TN-S supply to allow for in calcultating the fault current, touch voltage, and disconnection time, therefore...)
Yea, but, when I earth my metal gas pipe and metal water pipe, like the good little boy I am, does this then make my house installation partly TT, thus potentally unsafe as you describe! (This is why I asked :-) )
Yep, I did, however, the site doesnt have an option for SWA, so I assumed if it was OK with the 4mm+1.5mm earth, then is must be OK with a 4mm earth!
I am using three core SWA, the armour is connected only at the house end. However the thrid earth conductor from the shed -> garage is not connected (as it is earthed via a rod)
Yea, the CU in under the stairs, in the middle of the house - the run was a pain in the arse, goes up to the loft, then back dowm - however I should be able to shorten this soon as the kitchen is being ripped out. (New SWA will now be run under the patio, not overhead with 8 coax, 5UTP and a couple alarm cables (It looks crap, but it was the only way without digging uo the crazy paving (Yea, I know, its all been dug up now!)))
Same excuse as before ;-)
Yea!, See! I was paying attention :-)
Nope, we are not, but the calcs were assuming I was, so I thought with a better earthing arrangement, it was even better than OK!
Okay, if I was to connect the garage to a 20A MCB, connected to a 30mA RCD in the shed (No not Ideal I know!), would I still need to calculate from the house?
If not, what would you suggest I do (I Really don't want to have to replace the cables!!)
With an RCD at the shed end, we can ignore earth loop impedance stuff, as the RCD will cut out given an L-E fault. You're left with the voltage drop needing calculating for the full 50m run (and the 4mm T&E figures are reusable for 4mm SWA, neglecting differences in temperature rise); if you do shorten the CU-to-shed run, and since you don't 'really' run heavy stuff in the garage, you're likely to be OK (but a calc wouldn't hurt).
(Another reason I ask this, is, if a generator is installed, this has to have a rod (AIUI), but if this has a rod, connected to the house earth supply, does this them break the earthing rules on my TN-S system?)
Transfer switches only seem to switch the L&N (Via an "all off" position)
S'far as I understand it... for one thing, there's a balance of risk: the possible differences in potential between the supplier's earth in a TN-S setup and the 'local' earth of buried metallic services doesn't arise in 'normal' conditions, but rather in the case of faults, while keeping all your inside-the-house pipework at the same potential protects against likelier, local faults. ('Better still' from this p-o-v is for the incoming services to be non-metallic, so it's just the within-house pipework which is bonded.) A second factor is how low a reistance the local earthing has: the connection to earth that you get from a substantial length of buried cast-iron water main and gas supply is usefuly lower than that from one lil' earth rod.
But I've neither read deeply nor thought hard about this, so I'd welcome anyone better-informed weighing in...
I haven't been following this thread that closely, but the recent questions seem to be ...
An installation can't be partly TT. If there's a metallic earth path back to the substation transformer then it's TN, if there isn't then it's TT. (IT doesn't apply here.)
A generator installation has to have its own means of earthing, in case the connection to the supply distributor's mains earth has been severed. The earths can be connected in parallel - there's no need for switching.
Here's a generator wiring diagram I've posted before:
L o----------------o \ SW1a Mains o----------> L to CU from meter +-------o | SW1: DPCO changeover switch | 100A break before make! N o----------------o | \ SW1b | o----------> N to CU | | +---o | | | | Supplier's E o-----------------------------o Main earth terminal earth | | / | | | | | 16mm^2 | | | earth | ------- | | | | | G.P. | | G.P. = Generator | | | protection, ------- | fuse or CB | | | plus 100mA RCD | | | -- | - | -- | | G( | | | | G( | | | | G( | | | G = generator winding | G( | | | | G( | | | | | | | | Generator | +---.B | | frame -> | | | | B = bonding connection -------.-- | B|\ | | \-------------+ | | | ----- --- Your earth electrode(s) - Re < 200 ohm
NB 1. You are supposed to consult with your electricity supplier / distribution network operator before installing switched standby generating plant.
NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if the generator is not rated to supply the whole installation.
Okay, the origanal question was why shouldt I connect the house earth to the garage TT earthing arangement - why must they be kept seperate? (The cable run up to the garage is about 50m in 4mm SWA)
So it has to be impossible to switch on too much? or can there just be some instructions to turn off some MCB's?
Well if you connected them together it would no longer be TT, but nobody should have said that isn't allowed. It depends what you're intending to do in the shed and whether the equipotential zone concept can be properly applied there. If you search Google groups for "shed electrics" you'll find plenty of previous threads to muse over.
What it says is "Means shall be provided to automatically disconnect such parts of the installation as may be necessary if the capacity of the generating set is exceeded."
(It's the garage, the shed uses the house earth, as it is close to the house) In the garage, the power is just for a couple of flurecent lights, a 12v PSU for some CCTV cameras & alarm sensors and a few sockets for general use (battery chargers, lead lamps, very occational DIY tools like drills/ jigsaws possibly a pressure washer once in a blue moon.
There isn't going to be any fixed appliances, or anything high demand.
Would it be better/safer etc, to have an earth rod in the garage, AND connect it to the house earth (Via a 4mm conductor)? Or just use the house earth Or just use the rod?
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