TT system scenario - Thoughts please ?

[AA410]

I only have the one RCD tester the 100 ma is a MK 80A 100ma i cant get back to this job until next weekend the installation consist of a small F/B at the supply 100ma RCD time delay then 40A mcb 16mm 3 core SWA 80m to feed a shed small F/B 30ma rcd to supply general power and lighting spiked at each ends

 

[Chr!s]

Why would you want to test at 500 mA ? these testers wont allow touch voltages over 50 volts anyhow.

50/ 500 mA = 100 ohms max even if the meter allowed this test to be carried out.

Or a 300 mA x 5 1500 mA = 33 ohms lol

Regards Chris

 

[AA410]

OK so if you are installing a 100ma time delay on a sub main what do you put on the test sheet x1 reading or x5 reading ordo you not test at x5

 

[Chr!s]

There is no requirement to test at x 5. Not sure about your meter, you would normally select 100 mA (S) selective/delayed.

Try this: electrical.------.org/wiring-matters/15/insp-test-rcd.cfm?type=pdf

Regards Chris

 

[AA410]

Thanks Chris thats a great help does my instalation sound right i have not installed one of these before the instalation is for a mate of mine a big thanks for your advise

 

[Chr!s]

Thanks Chris thats a great help does my instalation sound right i have not installed one of these before the instalation is for a mate of mine a big thanks for your advise

Sound ok, you need to meet the .2 disconnection time, you have a 100 mA time dalayed so to ensure a .2 disconnection the Zs needs to be less than 1150 ohms, i assume theres no issues there. So thats Earth fault protection sorted out.

You have a 40 Amp mcb protecting the live conductors so providing In is lees than your Iz and In is more than your Ib i see no issues.

Obviously you need to consider your volt drop.

Regards Chris

 

[AA410]

Thanks Chris the run is 3 core 16mm s/w/a 80m run and i made a boob its a 32A not a 40 A and V/D is 7.42 it looks ok

 

[Jimmyb]

lighting max 6.9v ?

 

[Richard Burns]

If you are protecting with a 32 or 40 A MCB then I am sure your design current will be less than 30A which would bring your volt drop below the 6.9v required for lighting at the CU. But I am not sure how long the lighting circuit may be inside the shed.

 

[AA410]

Thats something for me to look at the supply is for lighting and power the shed is on the large side the lighting is going to be in plastic conduit with 6x vapour proof fittings with a run of about 20m with a V/D of about 3.52v

 

[spark 68]

Hi,
Just a quick point on VD.

Reg 525.4, states that a greater voltage drop than stated in table 12A (app12)
(neglecting the motor circuits etc. part of this reg for the purpose of this discussion) may be accepted, provided that it is verified that the voltage variations are within the limits specified in the relevant product standard for the equipment, or in the absence of a product standard, in accordance with the manufacturers instruction.

So maybe the VD may not be so rigid, depending on the manufactures of the lighting, ie. if the lamps are rated at slightly less than the more common 220V-240V, Iam not suggesting trying to bend the rules by any means, but my interpretation of this reg, was it was meant to stop incorrect functioning of equipment, as always common sense applies.

Just a thought

 

[Richard Burns]

I am not sure I would like to try and justify a lower rated fitting, but might be OK.
I think for the lighting circuit even if you have 100W fittings you are only running 2.6A and have an inside shed VD of 1.5V (if using 1.5 cable). The 16mm is running well below its CCC and so there should be a lot of leeway there, I think if installed it would probably meet the requirements by measurement.

 

[spark 68]

Hi Richard,

I was not suggesting anybody break the regs with regards the 3% VD where lamps are concerned, but the regs are just guidance, and there is provision in the regs for leeway within reason (Reg 525.4).

The main reason for that reg. is for correct functioning of the devices (Lamps), and the other parts of the VD regs err on the side of caution to guarantee that no matter what devices are fitted, they will function correctly, which is good design practice, especially where the designer has no say in what may be fitted (empty building).

Now I would always try to design to the regs, but I would not get to hung up about it if I was a Volt or two out, and just note it as a departure, with the proviso that the lamps etc. started correctly and there was no adverse effects on the reliability etc. and my current capacity was not exceeded

I would be concerned if they were quite away out, and were borderline starting etc..etc., again just common sense applied rather than slavishly following the guidelines with no actual thought as to why. (I'll put my tin hat on now, LOL) :cyclist:

 

[AA410]

Is this correct the v/d is at 3% so at 80m 16mm s/w/a supply and 20m 1.5 singles 6A mcb it comes in at 5.7v is this right ?

 

[spark 68]

16mm = 2.8 mV/A/M

(2.8*32*80)/1000 = 7.168 V

3% of 230 = 6.9V

This already exceeds the 3% drop without the 1.5mm run.


I make it 10.65V in total, but this is calculated using the In of your ocpds, as Richard has already said your design current will most likely be lower than this, so you may be able to get it within bounds.