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Had a look at a couple of datasheets for RCBO's and they only say Type B curve. No typical/min/max disconnection time for the magnetic part of the breaker is specified.

Just looking to find out where to select 't' the disconnection time for the Adiabatic formula. (This is for college not an actual install.) Using an EN60898 breaker, 16A. Type B. Do I look up a datasheet for the breaker? Do I use table 41.1 in the regs book? Part of regulation 411.3.2.2 (0.4...

Did you manage to do that? Our lecturer hasn't managed to yet....

Yeah, that's what I was expecting.

In the pictures, I can't see a primary and secondary winding. I expected to see something more like this:

Was in class today and googled 3 phase transformer. These 2 images came up and I can't see that they would do much ? The C&G handout on transformers image is also shown.

Ha! Yes I know you wouldn't have a 2.5mm2 radial with a 32A load but just picked some numbers, anyway back to the original question which is how does C&G calculate cable size and voltage dropfor a ring final? In the second example I gave, from a C&G task: Ring final, design current 20A, breaker...

Just to plug the numbers in: 2.5mm2 drop per the OSG is 18mV/A/M The drop on radial of 20M with 2.5mm2 with 32A load is 11.52v If we did a ring we'd have 40M of cable (assuming same run for the return) BUT we're measuring the drop at the halfway point AND the wires are parallel so we use a...

OK, think I have it, finally! The ring doubles the cable length and halves the resistance so your mV/A/M drops to a quarter the book value. Thanks for sticking with me!

OK, so let me see if I have this right, per metre, per amp, there's 9mV drop in the live and 9mV drop in the neutral. For a ring, you have another pair in parallel, so the volt drop is half on each side. 4.5mV in all 4 wires, per amp, per metre. 9mV in total. Half of the 18mV. The length of...

So what you do is, get a figure for It, look up a cable size that has capacity of equal to or greater than that, find the voltage drop, calculate it, check against limits (<11.5V) and if bigger, go to next cable up. That's how you get the numbers. Regarding resistance, when you say quarter, you...

I think I have above, but I have a few instances of this issue. Basically we are given a design brief, such as: Fill out a design grid showing the correct cable sizes for all the circuits required in the new student accomodation block, shown in the attached plan. The design grid has each...

Yes, they do, but not as far as C&G is concerned! This is what I'm trying to get to the bottom of. We're being taught the process of cable calculations which is great for radials and lighting etc but doesn't work for Ring Finals for obvious reasons, however there is nothing in the course notes...

So, C&G's answer to this question is 4mm2 and how did they come to this answer? Well, it appears that for radial and lighting circuits, you use the breaker rating plus any factors to get It and then look up the relevant installation method. For a Ring Final, you take the Ib, the design...

Alas putting "No sums necessary" would not be the 'right' answer as far as the C&G exam marker is concerned... This wouldn't be the first exam question I've read that has a 'correct' answer that is very dubious... At least it's not me being thick. Somehow I just need to figure out how C&G come...

Well another week passed and I still don't have a proper answer to where the process is wrong. To reiterate, this is not a question for a real life situation, its a C&G test question. I fully understand that 2.5mm2 is the norm for Ring Final and that the regs book says that as long as there a...

Just found that the OSG does have appendix H for Household and similar installations, which shows 2.5mm2 for a ring. However this example question is not a house, its a shop. A PC shop and the ring circuit is in the repair room. No idea if that has any bearing on the matter though!

Thanks guys. I'm still not sure what C&G are getting at, slipping a ring circuit into the question, which is a set of 7 circuits, all radial except one. It's in the form of a grid sheet. The fact that each circuit in the grid has the same sections for filling in the Ib, In, It, CSA etc etc...

Thanks for responding! The question specified 0.8 for the grouping cable factor Cg, 1.0 for the temp factor Ca. 1.0 for Cf. No other factors needed. It also specified method B. The example is t&e in dado trunking. We were told to reference tables F5i and F5ii. Table F6 being for other...

Just started Level 3 C&G and we're doing cable sizing calcs. Using a previous years exam question. We ran through a few examples of radial circuits, all well and good. Following this proceedure: Work out total load on circuit from plan. Work out design current. Ib. Ib<In<It Find suitable...