As per previous posts on this 'project' tew1 only sees and hears what he/she wants to hear and see (hence we will end up with possibly 34 circuits for this house).
I doubt a qualified electrician or the LABC have been anywhere near this property.
Submain Cable to house extension
- Thread starter tew1
- Start date
I doubt a qualified electrician or the LABC have been anywhere near this property.
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In this case, I agree that the submain is totally unnecessary. However, more generally, submains are often unavoidable, even when the supply is TN-S. If the Ze of a TN-S is such that the 0.55Ω Zs for a BS1361 fuse (and also maybe even the 0.89Ω Zs required for a BS3036 fuse) (with any cable) cannot be achieved, what would you do - rely on an RCD to achieve the required disconnection times?
Kind Regards, John
The house was rewired by a qualified electrician a few years ago. Due to the size of the property and layout, it was not practical or economical to extend the original ring mains or lighting circuits, so we decided to run separate radials, which is why there are so many circuits. It's actually brilliant, because if one lightbulb blows or something trips, the whole house isn't cut off. Regardless, I want to have the extension wired to a separate local CU because it is going to be an annex with separate access. So, how it is possible to run a submain? Can it be put on a 60a RCD to achieve the correct disconnection time and Zs value? (I won't attempt to calculate this myself because it would be a first). And how is this any different from having long meter tails coming from the meter box through a property to the CU?
Even if I'm not doing the work, if you can bare with me, I like to understand these things anyway, because, from experience, alot of 'qualified' tradesmen don't actually know what they're doing, and sometimes it's good to know whether someone is fobbing you off or doing a bad job to save some money. I didn't think it was necessary to call out an electrician just to determine what type of cable to run. In self build projects, it's common for non-qualified individuals to chase walls and run suitable cables to and from locations because it is a time-consuming and mundane task, and only necessary to pay a qualified electrician when it comes to terminating, installing devices e.t.c.
Even if I'm not doing the work, if you can bare with me, I like to understand these things anyway, because, from experience, alot of 'qualified' tradesmen don't actually know what they're doing, and sometimes it's good to know whether someone is fobbing you off or doing a bad job to save some money. I didn't think it was necessary to call out an electrician just to determine what type of cable to run. In self build projects, it's common for non-qualified individuals to chase walls and run suitable cables to and from locations because it is a time-consuming and mundane task, and only necessary to pay a qualified electrician when it comes to terminating, installing devices e.t.c.
It isn't any different - hence the question I posed to flameport. He is right in saying that it might well not be possible in a TN-S installation to get a low enough Zs for a 60A BS1361 to provide the required disconnection times (with any size of cable), but many people with TN-S installations cannot avoid having submains (or, as you say, merely 'long tails'). As I said, the only solution I can think of would be to use an RCD to achieve the required disconnection times (a time-delayed RCD would be fine, and would give discrimination from any RCDs in the CU fed by the submain).
I'm not convinced that many electricians think of this, since, even in my limited experience, I'm seen plenty of submains/long tails protected by 60A (or even 80A) BS1361 fuses (and no upstream RCD).
Kind Regards, John
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If the rewiring was done by a professional then you will have an Installation Certificate stating the Ze then you can calculate if the submain is satisfactory.
You also forgot the most important part of designing, testing and registering the work with your LABC of course.
But the point you have been missing since you started this project and continue to either ignore or not understand is that your electrician cannot do what you ask without at the very least breaking the rules of his competent persons scheme and more probably the law.
For the sake of repeating myself, you MUST involve the electrician at the outset to DESIGN the circuit (that means the type, size and route of the cable). That doesn't stop you negotiating to chase walls etc.
Otherwise you must inform your LABC immediately and before you start the work.
They never got it to me. The electrician was a subcontractor to the main builder, the builder probably still has it in a drawer. At the time, I wasn't aware that you received a certificate for electrical installations. The CU has a label stating the installation has been tested, the date, the company e.t.c. so I might need to give them a call about it.
Can I ask, what does the disconnection time relate to? I understand Ze refers to the meter incomer, and Zs is at the end of the submain, but 5 seconds seems a long time for a cable to short before it's protective device trips. Does this refer to mechanical damage? (it would be more than 50mm above the plasterboard, and in conduit where it comes down to the CU). Can you protect a 'submain' with a 30ma RCD, then just install MCB's in the submain's CU?
An RCD could be used - but to install a new circuit on a TN system knowing it requires an RCD to meet disconnection time would be a very poor design.
When current flows through cables, they heat up. When a fault occurs, the fault current causes them to heat up significantly. If the supply is not disconnected in a suitable time, they will catch fire and burn your house down.
If 5 seconds is too long, shorter disconnection times can be achieved by altering the design. In many cases, much shorter times would be required anyway.
A single 30mA RCD for the submain with MCBs in the CU does not comply, since all of the CU circuits will be disconnected in the event of a fault. If an RCD had to be used (due to there really being no other option), it most likely would be a time delayed type, and not a 30mA one either.
A smaller value fuse could be used.
Other types of fuse and protective device are available and could be used - provided that discrimination with downstream devices can be ensured.
The measured value of Ze could be used, rather than the assumed one. This would require more regular testing than then usual 10 years to ensure the value of Ze had not changed.
In some unlikely situations, supplementary equipotential bonding could be used.
More than one circuit could be installed, such as extending the original circuits into the extension.
If this is a separately accessed extension, it could have it's own metered supply installed by the DNO or whoever.
The point of this is that once you move away from the small number of 'standard' circuits as typically used in domestic properties, there are many things which must be considered when designing circuits. It is not appropriate to shove cables in and hope, neither is it of any real use asking random questions, since it is not possible to ask questions about things you are unaware of.
Are you saying that you don't feel that there is any way to install a ≥60A submain in a TN-S system which does not represent 'very poor design'? - even a Type B MCBs would be no good if the Ze is anywhere near the maximum possible with TN-S.
Kind Regards, John
ok, well, whatever happens, I need a submain because it needs to be separated from the rest of the house (metered or not), and a 10mm supply cable is needed. The CPC in the T&E is more than adequate as there is no supplementary bonding required to the structure or plumbing. As and when I come to building the extension, the electrician can do the calculations and decide on a solution using the available cable to meet the required disconnection time using one of the solutions you've mentioned (flameport)
I don't get why this is a problem. There is no issue with all the circuits being on one RCD. It's only a bedroom, bathroom and garage. There is no kitchen, the smoke detectors will be connected to the existing house and I could use the existing garage lighting feed from the house to connect one light in the stairway for safety.
The 'problem' is that current regs require that installations should be divided into circuits "to avoid danger and minimsie inconvenience in the event of a fault". This is commonly interpreted as meaning that all circuits should not be protected by the same RCD - hence the standard current practice of having a CU with two RCDs, each protecting a bank of circuits.
The main 'danger' usually cited is that (if everything is protected by the same RCD) a fault on one circuit will take out the whole installation, including lighting circuits, and that this sudden loss of light may result in danger (tripping over, knocking over pans of hot water/oil etc.). However, that danger would equally be present if the lighting circuit itself developed a fault (unless there were lights actually plugged into other circuits and 'ready to go') - but the regs require no provision for avoiding that hazard ... so go figure
Kind Regards, John
yep, sure John I understand that. But in this case, I think we can treat it like a submain to a garage/outbuilding, and protect it by a 30ma RCD with MCB's protecting the individual circuits downline. Reason being, I can run the lights in the stairway (with the door to the garage) from the existing house circuit so you're not plunged into darkness. Also, as it is just a bedroom, bathroom and garage, there is no normal instance where an earth fault would occur at night. In my experience, the only time RCD's have tripped in the house is from kitchen appliances (toast touching the heating element, the blender leaking e.t.c.) or when you do some wiring and the neutral touches a metal casing. Most people don't trim hedges in the rain at night, and even if they did, garden appliances often have integrated RCD's in their cord plugs. Blown light bulbs trip the MCB's only. Sod's law also means that the electrician has put the lights on the same side of the split-load board as the plug sockets, so it defeats the object of split-loading the board for separate RCD protected circuits in the first place. Strange there isn't any requirement to separate circuit types according to use on a split load board. hmmm.
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