Fuse selection

[Big_Spark]

What is the difference between the fuse Zs and the circuit Zs (Ze+R1+R2)?, why or when do we use the fuse Zs.
Albert

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By this I presume you are talking about the Zs values as listed in Tables 41B1, 41B2 and 41C.

The Circuit Zs, as you call it, is the Earth Loop Impedance of the Circuit, however the Fuse Zs, as you call it are the MAXIMUM Earth Loop Impedances allowable for circuits using those devices as listed as the Header of the table clearly indicates.

 

[Albert]

I presume you are talking about the Zs values as listed in Tables 41B1, 41B2 and 41C.
The Circuit Zs, as you call it, is the Earth Loop Impedance of the Circuit, however the Fuse Zs, as you call it are the MAXIMUM Earth Loop Impedances allowable for circuits using those devices as listed as the Header of the table clearly indicates.

Yes this are the tables. As far as I know the maximum recommended value for the Earth Loop Impedance for the circuit is 0.5 Ohm, If I make sure that the test result is always max. 0.5 ohm by this (correct me if I am wrong) according to table 41B1, we are safe up to 50A. What happens if the requirements are much lower than 0.5 ohm, like in table 41B2, how do we overcome this (do we need to increase the csa until it meets this value? (no I am not trying to be difficult I am honestly trying to understand

 

[Big_Spark]

Albert, which edition of the Regs are you reading??

According to table 41B1, section (a), fuses to BS88-2.1 and BS88-6, the maximum permissable Zs for a circuit with a 16A fuse link is 2.82 Ohms, so where are you getting this 0.5 Ohms from?

 

[Albert]

OK!!!, sorry I should explain better, I'm confused between the R1+R2 which are the value that you will get when you testing the cpc continuity, as far as we were told this value should not exceed 0.5 ohm. If you add to this value the Ze it will give you the Zs of the circuit (to find Ip you will use this Ip=Uo/Zs). I'm afraid that (probably) I'm mixing tomatos with oranges, this was my original question; as we have again a Zs which related to the fuse and the values are in table 41B1 (I have the correct addition 16th). You are right the value for BS 88-2.1 , 16A is 2.82 ohms. I do not understand why if the max first Zs should be about (this figure was repeatedly mentioned to me in the course) 0.5 ohm, how come that the fuse values allow more, unless there is no relation between the two and it's a coincidence that they both are called Zs, or as it happened in the past the teacher does not what he is talking about... and so am I!!!
[/quote]Albert, which edition of the Regs are you reading??

According to table 41B1, section (a), fuses to BS88-2.1 and BS88-6, the maximum permissable Zs for a circuit with a 16A fuse link is 2.82 Ohms, so where are you getting this 0.5 Ohms from?

 

[Big_Spark]

Albert, before going further, do you actually understand what R1, R2, Ze and Zs are?

 

[Albert]

I think so, R1 is the resistance of the phase cable from the enterance to the cu to the load, R2 is the resistance of the cpc from the enterance to the cu to the load, Ze is the resistance of the phase between the ccu and the transformer (the value normaly given by the supplier). Zs its the total resistance of the three (Earth fault loop Impedance) all in IEE on site page 79 and page 157. it ia possible to find the tabulated values of R1+R2 in table 9A page 158 when the values are in mohm per meter.
is it near to what it really should be?....

 

[Big_Spark]

Albert, here we go, get your thinking hat on fella and put your brain in gear..bit unfair on a Friday night, but you asked the question

You have a circuit wired in 2.5mm2 T&E, it is 20m in length and protected by a BS88-2.1 16A HRC Fuse. The Regs state this protective device must operate in 0.4s in order to comply.

From Table 9A (OSG-P158) we see that 2.5/1.5 has a R1+R2/meter value of 19.51mOhms per meter.

So, 19.51 x 20 / 1000 = 0.39 Ohms.

In your original post you state that Zs = 0.32 Ohms, I am now questioning where this assumption came from, as you can see R1+R2 is greater than that.

Now I will assume that what you actually meant was Ze, which would mean that Ze = 0.32 Ohms.

That being the case Zs = Ze+(R1+R2) = 0.32 = 0.39 = 0.71 Ohms

Therefore the Earth Fault Loop Impedance of this circuit is 0.71 Ohms.

Now from Table 41B1 (BS7671 -P45) section (a) we see that in order to comply with Regulation 413-02-09 and 413-02-10 the maximum permissable Earth Fault Loop Impedance for the Overload protective device in this circuit (16A BS88-2.1) must not exceed 2.82 Ohms.

Therefore as 0.71 is LESS that 2.82, this circuit will comply with the Regulations.

If you wish to then discover the fault current Ipz = Uo/Zs = 230 / 0.71 = 323.94A.

Then from graph Fig 3.3b (BS7671-P198) we see that this fault current will operate the protective device in less that 0.01 seconds, well within the 0.4 seconds stated in the Regs. In fact the graph indicates that the protective device will only require a Fault current of 85A in order to comply with the 0.4 second rule.



Please don't tell you don't understand that!!

 

[Albert]

Albert, here we go, get your thinking hat on fella and put your brain in gear..bit unfair on a Friday night, but you asked the question
Please don't tell you don't understand that!!

That was nice, give yourself a star sticker , yes I meant Ze and it is a figure that I picked for the exercise.
the funny (or the sad) thing is that I did not explain myself properly, because the way you answered this question was the way i did but with a difference, you understood what you were doing and I did understand only part of it, I did not see the place of the Zs value from the fuse table (41B1, 2.82 ohm in this case). And it makes me angry to think that in class we covered all this issues of calculating R1, R2 etc. but not even once it was mentioned what it is the fuse value, Zs for.
Thanks to you it is really clear in my mind.
I would assume that if the value of Zs of the circuit is higher that the fuse value in the table we suppose to increase the csa of the cable so to reduce the resistance.
Have a nice weekend
and many thanks again
Albert

 

[Big_Spark]

Albert, the value in those tables 41B1 etc are not FUSE values. It may be easy for you to think of them that way, but I think that is where you were confusing yourself too.

These values are the MAXIMUM PERMISSABLE Earth Fault Loop Impedance (Zs) for a given type and rating of overload protective device, the different tables deal with the differing types of devices.

If you get it into your head to only think of them as maximum Zs, you will actually get to grips with dealing with these issues easier and prevent yourself from getting confused about the values.

You are correct, if the value of Zs exceeds the Maximum permissable then you must uprate the cable.

You need to bear in mind that is the Zs is too high, then the phase and neutral conductors are likely to be under-sized too, a simple volt drop calculation should confirm this.

Normally, and this is worth remembering, if you calculate the maximum load the circuit will normally be subjected too, then ensure that the volt drop does not exceed 6V over the length of the circuit, then you will certainly find that the value of Zs will be well within tolerances.

The only time this may get complicated is with large cored SWA's (50mm2 and above), were the cores may comply, but the armouring will not give compliance, so the cheaper alternative is to run a seperate conductor in to act as the primary earth path for the supply.

 

[Albert]

Thanks,
Yes I understood it but explained badly, when I said that this is the fuse value I meant what you said, that this is the max value of the Earth Fault Loop Impedance permitted for a specific selected fuse.
Voltage drop never was a problem, basically the permitted value is 4%, (9.2V for 230V supply). One thing (of many) that you did not see where it came from is the value of 0.5 ohm as the max. permitted value for the R1+R2 (pcp continuity) of a circuit, this figure was repeated many times to us and I am 100% sure about it as the value, but I am not sure now whether it is correct or another one of those unintelligent bits that you get in the class...
Just a remark:
I read in one of you replys about conduits and trunkings, you mentioned the 40% and 45%., which is the permitted space can be used when you want to pass a cable in a conduit or trunking. This is the old system and as far as I now it is still valid if you wish to pass T&E cable type or for special cases. For single core cables we use tabulated values that can be found in OSG pages110-114 (tables 5A,5B,5C,5D, 5E and 5F). Don't tell me that I miss understood this one as well! (I will jump of the window of the ...ground floor)

 

[Big_Spark]

Albert, the Maximum R1 and R2 reading is a recommendation rather than a hard and fast rule. I do not believe, though may be wrong, that it is written down in the regs book, I certainly cannot find it.

I think it would be a bad design to let this value rise above 0.5 Ohms because then there are other implications to consider, like volt drop, disconnection times etc

The reason I said you do not want to go over about 40 to 45% was referring to a general principle, when you do over that actually drawing cables into conduit, around bends etc can be a pain in the neck and you risk stress damage to the cores.

If the conduit run is straight then there is no reason to take the grouping higher. Different sparks/engineers do things differently, so long as whatever method you use complies with the Regs and would be considered good design, there is no problem.

 

[Albert]

Albert, the Maximum R1 and R2 reading is a recommendation rather than a hard and fast rule.
The reason I said you do not want to go over about 40 to 45% was referring to a general principle, when you do over that actually drawing cables into conduit, around bends etc can be a pain in the neck and you risk stress damage to the cores.

I thought that the 0.5 ohm could be a recommended value, but it did not sound like that when it was passed to us..., no reason was given and it was said that this is a value to stick to, in one of the assignments I had to explain what would I do if the value is 0.6 ohm...? this does not make a principal and significant difference from 0.5 ohm does it?. I searched for this value everywhere and could not find it.
Now after you kindly and with much patience explained to me, I understand what are my targets and build my circuit accordingly.
(By the way, I am sure that you noticed that table 5D page 112, recommends cable factors for bends as well)

 

[Big_Spark]

Yes Albert I was aware of the cable factors for bends.

I'm a bit rusty on some of this as we use cable calc software for speed when designing and pricing jobs..I have enjoyed getting to grips with this manually again...nice to know the brain still functions!

The only way to know if the 0.6 Ohms makes a difference is to create a circuit from scratch, just invent it, and do the calculations for Volt drop, Zs etc.

 

[Albert]

The only way to know if the 0.6 Ohms makes a difference is to create a circuit from scratch, just invent it, and do the calculations for Volt drop, Zs etc.

Yes I know, and this is what made it to look so important, the question wan not related to any specific details, so it was not possible to check the other values (volt drop, the fuse permitted Zs etc.)
In any case many thanks again, and I'm sure that we will talk soon (I'm still in my learning curve...).