Significance of 2.18ohns

[John-]

Hi. I have Metrel MI 3152 multifunctional tester and in the Line, Z auto menu there is a setting for Zlim(Z (LPE),Z (LN)) and it is fixed at 2.18 ohms can’t be adjusted. I’ve just done a job on a site and the prospective short circuit fault current is very low and really limits the length of and size of conductors we can use. And testing an existing circuit shows that there the 20A C-type breakers they have in now will take 13-14s to trip, where as a B would be .4 or less. This is a remote TT building from a TNCS supply with 100mA rcd, so fault protection is okay, quite a way from the supply. So I thought I’d use my tester just to drop back from C to B, rather than look at the graphs in the Regs, to see if reducing the type from C to B would satisfy but it seems that the 2.18 ohm limit is a significant factor in the tester, its fixed in red and won’t let me circumvent and conciquentky states fail even though tegs say fine. Am I being thick here what am I missing? My thoughts were looking at the current characteristics graphs in the regs I could reduce from C to B then swap the C the RCBOs for Bs because they’re not using inductive loads on most of the affected circuits and the B type curve would work with the 104A PSC I have on the end of one if the circuits / 150A in the board. But the tester says fail! Because of the 2.18 ohm.

Any thoughts please?

Cheers
John

 

[westward10]

Whether B or C the rcd provides fault protection there is no need to change them.

 

[John-]

Whether B or C the rcd provides fault protection there is no need to change them.

Thanks for the info, RCD will provide for protection against line or neuteal to esrth faults. It will not offer any protection in the event of a Short Circuit or overload between Line and Neutral, it’s this area that I am concerned about - operation of the overload device in .4 or under in the event of a Short Circuit to live and neutral not live to Earth.

 

[Julie.]

There isn't a requirement to operate within 0.4s for l-n faults, only for ads although if it doesn't operate fast enough then the cable can overheat under fault condition, but you can just check that given the trip time, fault current and cable type (copper/pvc).

Thanks for the info, RCD will provide for protection against line or neuteal to esrth faults. It will not offer any protection in the event of a Short Circuit or overload between Line and Neutral, it’s this area that I am concerned about - operation of the overload device in .4 or under in the event of a Short Circuit to live and neutral not live to Earth.

 

[Vortigern]

Just a guess or stab in the dark here, do you have a setting for what Amps MCB/circuit you are testing? Looks like yours is set for 16a value. If you change that, the fixed EFLI value should change with it. If it is TT then are we looking at 0.2s?

 

[John-]

Adiabatic you mean? e.g ebdure the t from k2 s2 / i2 is greater than the t on the current characteristic graph for the device?

 

[John-]

Just a guess or stab in the dark here, do you have a setting for what Amps MCB/circuit you are testing? Looks like yours is set for 16a value. If you change that, the fixed EFLI value should change with it. If it is TT then are we looking at 0.2s?

I was just about to reply of course I checked that… Well I thought I did… Yes you are right value changes corresponding to the value of the device sorry, feel like such a plumb now….

 

[Vortigern]

Easily done mate, anyone could do it.

 

[timhoward]

I was just about to reply of course I checked that… Well I thought I did… Yes you are right value changes corresponding to the value of the device sorry, feel like such a plumb now….

I have one of those testers and I tend to leave that option off so it cant confuse me. (Just change it to blank.)
I find it a very good tester that was intended to be my backup but it comes out to play quite a lot especially when fault finding.

 

[Julie.]

Adiabatic you mean? e.g ebdure the t from k2 s2 / i2 is greater than the t on the current characteristic graph for the device?

Yes

But using the actual disconnection time from your Zs, 95% voltage and when hot.

 

[John-]

Yes

But using the actual disconnection time from your Zs, 95% voltage and when hot.

Thanks for coming back Julie. So i took the measured PSCC at the end of the circuit, 105A. K of 115, CSA 2.5mm
7.49seconds. Looking at the current graph, for a 20A C type MCB is around 18s. It will overheat. Correct? If i downgrade to B type, it,s less than .1. How would i apply the worst case scenario’s you describe?

 

[Julie.]

Thanks for coming back Julie. So i took the measured PSCC at the end of the circuit, 105A. K of 115, CSA 2.5mm
7.49seconds. Looking at the current graph, for a 20A C type MCB is around 18s. It will overheat. Correct? If i downgrade to B type, it,s less than .1. How would i apply the worst case scenario’s you describe?

The issue is to find the worse case, it may be when the Zs is 230V/105A - use the trip time for 105A - compare the time from the adiabatic to the trip time for 105A, but also when the same Zs is on 95% voltage, and hot (1.2x Zs) - which means actually a lower fault current, this would give a longer acceptable time from the adiabatic, but would also result in a longer time to trip.

You would have to calculate both cases as you can't really predict which will be worse. Higher currents are bad from the adiabatic point of view, but result in quicker trip times, so actually can help overall, but in other cases the lower current allows longer adiabatic time, which may actually be better even though it results in longer trip times.

The problem with 105A though, I think it will be the same trip time (or very similar) for both type b and type c

So actually unless you can drop the protection down (16A?), then you must increase the cable size, which not only reduces the Zs, but increases the capacity via the adiabatic.