type C rcbo's

[Steve3948]

Not sure I follow Steve, I draw out Adiabatic lines for various cables, this allows me to see if the thermal constraints have been met for all conductors.

My bad then, I thought you were referring to t=k

 

[park1]

My bad then, I thought you were referring to t=k

 

[Steve3948]

I am

I know haha :innocent

At last we agree :slap

 

[Nicky Tesla]

John Whitfield......The Electricians Guide to the 17th Edition.........page 43..........3.6.5 Protecting conductors

The current at which the protective device operates must never be greater than 1.45 times the rating of the smallest cable protected.

For example consider a cable system rated at 30A and protected by an MCB type C, rated at 32A. Reference to Fig 3.17(time/current characteristics graph) shows that a prolonged overload of about 38A will open the breaker after about 10400 seconds (about 2 1/2 hours). The ratio of operating current over rated current is thus 38/30 or 1.27, significantly lower than the maximum of 1.45.

All circuit breakers and HBC fuses listed in {3.6.2 sections 2 and 3} will comply with the regulations if their rating does not exceed that of the smallest cable protected.

 

[park1]

Wrong tree again.

 

[Nicky Tesla]

Wrong tree again.

Show me your worked example and let me pick the fruits from your tree, but if they are sour i will spit them back out again:|

 

[park1]

Show me your worked example and let me pick the fruits from your tree, but if they are sour i will spit them back out again:|

You need to satisfy yourself that the thermal constraints are met, instead of referencing other publications, use BS7671 Chapter 43.

 

[Nicky Tesla]

No fruit for tea thenopcorn

---------- Post Auto-Merged at 22:01 ---------- Previous post was made at 21:50 ----------

How i see it, is that if your cable is rated at the same or higher than your MCB rating it will be ok. The adiabatic needs to be consulted only when your cpc is smaller ie less rated than your lines. So the lines should never have a problem.

But if im wrong then please enlighten me with an example, but if you cannot give an example please donot bother suggesting i read this and read that cos that just means that you are incapable of giving an example.:Neil and i am indeed correct.

 

[park1]

I asked for an example earlier but got ignored - to be or not to be ROTFWL I have even answered his Rod questions cos i was bored :slap

---------- Post Auto-Merged at 22:07 ---------- Previous post was made at 22:04 ----------

Forgot to say, its probably not only reading chapter 43 but the interpretation of it.....

(Then again after recent threads perhaps chapter 43 should say 43.1 If Cable is rated at the Current and does not get hot then its fine to use :slap )

I believe chapter 43 to be quite clear with regard to thermal constraints, I don't intend to do a worked example, but do feel free if your bored. .

2.5 twin earth, 20 amp type c mcb, fault current 115 amps, is the cable protected. ?

 

[Steve3948]

So 2.5mm

 

[park1]

Steve, chapter 43 deals with live conductors, the faults between live conductors.

 

[Steve3948]

Steve, chapter 43 deals with live conductors, the faults between live conductors.

And LINE conductors.

431.1.2 is of particular interest regarding the original post regarding RCBO

 

[park1]

A line conductor is a live conductor.

Your k2 S2 will differ.

Both cables are 2.5, and the k values may differ.

 

[Steve3948]

A line conductor is a live conductor. Your k2 S2 will differ.

Both cables are 2.5, and the k values may differ.

k

 

[Nicky Tesla]

115A that calcs to 2.087ohms total Zs

Lets say 0.50ohms for the r1+r2 of the ring and 0.087 for the Ze at the transformer. That leaves 1.5ohms for the supply cable from the trans to the DB. That means that at say 50A normal load the DB would have a supply of 115A x 1.5ohms less than 240v=172.5v

This is way lower than a realistic or allowable value.

Also it says in the regs that the t= adiabatic equation is for faults of < 0.1 sec only

---------- Post Auto-Merged at 22:25 ---------- Previous post was made at 22:22 ----------

115A that calcs to 2.087 total Zs............sorry forgot for a minute, its nawt to do with Zs is or Ze i meant L/N loop

 

[Nicky Tesla]

Well im disappointed with you all............so many of you come on here saying they are here to educate those that dont quite understand, for eg sidewinder(no offence) yet no-one is willing to show any working out for the above problem.

As i have shown above, the example of 115A for a short should not really exist due to volt drop regs, and it is L/N we are talking about and not cpc/Zs here.

I think the adiabatic is not that simple to use in equations without other factors coming into force...ie it states in the regs that the equation is adequately correct for trip times of less than 0.1 seconds only.

So come on, EDUCATE:Neil

 

[Sidewinder]

Well im disappointed with you all............so many of you come on here saying they are here to educate those that dont quite understand, for eg sidewinder(no offence) yet no-one is willing to show any working out for the above problem.As i have shown above, the example of 115A for a short should not really exist due to volt drop regs, and it is L/N we are talking about and not cpc/Zs here.

I think the adiabatic is not that simple to use in equations without other factors coming into force...ie it states in the regs that the equation is adequately correct for trip times of less than 0.1 seconds only.

So come on, EDUCATE:Neil

Nicky,

In case you had not noticed I'm not posting much, as I'm too busy.

I'll work out whatever you want, but,you'll have to cover for me in the mean time.

I have a Danfoss VLT to progamme & install Monday a.m. we have no motor or machine parameters and the drive is not a like for like swap.

You do this for me & I'll answer this thread for you, sorry, but at the moment I just don't have time.

I have to be in work on site by 07:00 tomorrow.

I will have about my 10th day (ish) off since the beginning of the year Sunday as the kids want to go out, and that includes not having weekends off, so in about 6 months I have had the equivalent of 5 weekends off.

Thus like it or not I don't have the time to put into delivering detailed answers to questions.

Plus I have also just been tasked with developing and delivering a level 4 course including all course notes and presentations etc. and getting the course provider through the assessment.

So, Nicky, like it or not, sorry, but I don't have the time to put the thought into any posts at the moment.

 

[welchyboy]

As a general rule on a tt system I usually measure the L-N external loop at the board (which is normally <0.35 ohms)then just add the r1+r2 results to it which are normally with t&e going to be higher than r1+n anyway , if these give you an ok reading below your max zs limit then ur fine

If they are over or a little bit tight then I measure loop for the circuit L-N but they rarely are, but yes with type C 's this could be a possibility

When designing circuits using the copper resisstance per meter table is all it requires really again plus your max L-N external loop at origin

 

[Nicky Tesla]

Does anyone have any decent links to adiabatic calcs. I have done them before but there is more to them than first seen.

I have searched for I2T graphs from manufacturers MCBs before but the graphs (if there are any) tend to be rather sketchy.

I realy need to get to the bottom of this. The adiabatic is not as simple as people seem to think.

 

[Manator]

It is very scientific, and use of wiki can show you the majority of scientific papers of the process. I have some reference books for earthing and electrical calculations and the majority of them even I can not understand as they were perceived by mathematicians and not electrical engineers.

most of the literature deals with pressure and heat and is from the Greek adiabatos, impassable : a-, not; see a-1 + diabatos, passable (dia-, dia- + batos, passable from bainein, to go; see gw- in Indo-European root).

I just go with the flow and use the calculation prescribed.