X/R ratio of MCBs

[Cookie]

What is the max X/R ratio of your MCBs? What are they required to be tested to? As I understand it no one calcs the X/R ratio of the circuit they are installing or knows the X/R ratio from the DNO. Multi function tester only gives ohms instead of R+jX.

 

[Julie.]

What is the max X/R ratio of your MCBs? What are they required to be tested to? As I understand it no one calcs the X/R ratio of the circuit they are installing or knows the X/R ratio from the DNO. Multi function tester only gives ohms instead of R+jX.

The interrupting capacity and requirements for all the switching devices are specified in the associated specification for that particular device, so for circuit breakers it could be bsen 60898 or 60947 or 61009 etc.

As far as bs 7671 is concerned, since this is limited to up to 1kV so in general is distribution circuits provided from a dno supply, there is a restricted range of x/r.

Therefore, 537.4 defines what equipment is suitable taking this into account, going outside of this guidance does require one to check the suitability - implied throughout section 5 and of course 133.2.

The most onerous duty is obviously at pi/2 which in general is under load conditions rather than fault this is in practical terms motors and lighting circuits, so typically bsen60669 which calls for 0.95pf, or 0.6pf or cfl or cfli type loads etc depending on the section.

In summary at low voltage the x/r need is pretty much removed from consideration for general installation practices.

However at higher voltages, or in some special applications we still need to account for it, as far as lv is concerned protective and switching devices have moved on a lot from the early days, so the need to calculate this sort of thing is much reduced.

 

[Cookie]

So basically inductive loads force breaking distances which exceed worse case X/R?

 

[Vortigern]

Well call me dumb but never heard of X/R what is it?

 

[davesparks]

I have no idea what the X/R ratio of an MCB is.

Presumably R is resistance and X is reactance?

 

[Cookie]

Yes, R is resistance X is reactance.

 

[Vortigern]

X:R would indicate ratio X/R indicates division. Anyway I still don't know how that applies, where or what do you do with it. 'scuse my ignorance but could you enlarge a little please?

 

[pc1966]

I think the OP's question really is what the worst-case power factor is that the type of MCBs used in the UK are rated to break.
[automerge]1596437631[/automerge]
tan(theta) = X / R

 

[Julie.]

The X/R in relation to MCBs is rather misleading, it relates to the capacity to interrupt the current, basically there are two aspects - firstly it is easier to switch resistive loads rather than ones of lower power factor (hence you may see 15A resistive/7A reactive type rating on some switches), the second is on large sites where there are many motors or similar (hence high X) when a fault occurs these motors contribute to the fault at the very early stages, however later on they can't sustain it, so a site with many motors will have a sustained fault level equal to that of the supply, but within the first few ms it would be considerably more - just at the point when a MCB/MCCB would be opening on a magnetic/instantaneous fault.

The international standards IEC/EN take this into account within the specification for switches, circuit breakers etc such that it must operate at appropriate power factors, and motor contribution as standard - so the switch example above under IEC/EN/BSEN would be a 7A switch, even though physically it could actually work adequately for larger resistive current ratings.

At medium and high voltage however this needs to be accounted for.

In the US, they follow this practice on to LV and rate their breakers quite differently, in general they test at a power factor of 0.5 (lower at higher current ratings) and assign the rating based on that. Then there are correction factors that they apply (without test) to rate the breaker for other power factors.

So under IEC etc a breaker may be able to open 750A at power factors down to 0.2, it must be tested over this whole range up to 1.0 and show it works effectively.

This same breaker may be sold into the US where it has a rating of 1000A, and under US standards it must be tested at a power factor of 0.5.

Now if you need to use the IEC rated one over unusual ranges - big local motor contribution etc you merely check the effective power factor is within the rating and the job's done.

In the US you would actually use an effective X/R - this corresponds to our use of pf, so for the same example a pf of 0.2 actually would be X/R = 4.9 and using a look up table then say the 1000A breaker should be suitable for 762A as the correction factor for X/R = 4.9 is 0.762


I think the actual requirements are now changing in the US, I think new ANSI standards are changing this such that MCCB are going to be rated at 0.15pf (X/R = 6.6) which is more in line with IEC stuff.

It should be noted that the specification is different for each piece of kit, for example a domestic light switch is tested over a different duty cycle than say a motor starter contactor. As I said in my earlier post you would need to check the appropriate standard covering the item of switchgear to see what the test parameters are.

 

[davesparks]

So it's a bit like the AC22, AC23 etc rating of a switch?

 

[Julie.]

So it's a bit like the AC22, AC23 etc rating of a switch?

Yes the Utilisation Category relates under IEC to the pf that switch is expected to operate on, so an AC23 would be for higher X/R ratios - i.e. motors, or sites where there is large motor contribution; AC45 for highly capacitive X/R etc.

There are far too many for my brain to remember and it mainly relates to 60947, the same thing (test conditions/suitability ranges) occurs across other standards for all electrical kit

 

[Cookie]

I think the OP's question really is what the worst-case power factor is that the type of MCBs used in the UK are rated to break.
[automerge]1596437631[/automerge]
tan(theta) = X / R

Yup. you got it!