Saturday evening discussion - Part 1 (of 1 or 2!)

[JohnW2]

It’s amazing what issues arise during a Saturday evening discussion over a few glasses of wine! ...

The manufacturer’s information for Wylex MCBs includes:

Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents when mounted in enclosures. It is good engineering practice to apply generous derating factors or make provision for adequate free air between devices. In these situations, and in common with other manufacturers, we recommend a 66% diversity factor is applied to the MCB nominal rated current where it is intended to load the MCBs continuously (in excess of 1 hour).

This is not crystal clear (at least, not to me - particularly their use of the word ‘diversity’, having previously talked about ‘derating factors’). Does it mean that if there are adjacent MCBs and the circuit in question is likely to be loaded for >1 hour, then the current through it should be limited to 66% of its ‘rated’ value (presumably its In)? If so (I’ll wait to see what some folk think!) the crux of the Saturday evening discussion will then become pertinent! What do you think?

Kind Regards, John

 

[securespark]

Good morning, fellow insomniac!

Very odd. Their catalogues show MCB's shoulder to shoulder.

I can see that breakers at or near their limit will generate heat and could trip an adjacent breaker through heat transfer.

I have never seen them installed with a module's gap between each, but maybe that's what we should be doing?

Wonder what other manufacturer's advice is?

 

[bernardgreen]

The ambient temperature around an MCB will affect the tripping point of the thermal trip. It would be interesting to know exactly how much the ambient affects the tripping point. Do the manufacturers publish this information in detail at various ambient temperatures ?.

False tripping of a 6 amp lighting MCB at 5 amps could have been due to heat from a hot MCB supplying an electric shower reducing the trip current . The lights only tripped their MCB when the shower had been in use by several people. The shower MCB was on its maximum rated current ( possibly overloaded ) ad it did occasionally trip. It was a badly designed installation, the shower was an afterthought and its MCB fitted next to the lights MCB. When it was replaced with a MCB and RCD in a separate CU the false tripping of the lights stopped.

The manufacturer's use of "diversity" may mean that if you expect a maximum continuous load ( for an hour or more ) of say 20 amps on a circuit then use a 32 amp MCB and size the cable for 32 amps or more if the MCBs are packed together in the CU,

 

[ericmark]

I am sure John like me you have had problems where near limit draw has caused MCB's to trip below their rated current when mounted next to similarly loaded MCB's. I can't see how any manufacturer could correct this with a thermal device.

I have questioned the leaving of the magnetic option in preference to the thermal method a few times. To me the old dash pot method of allowing a start up surge yet set a very close tolerance to overload can never be equalled with a thermal device.

There are now some very clever monitoring devices which do far more than the old dash pot system but the cost is something else.

In the main one can arrange MCB's so that two fully loaded ones do not sit side by side however the old idea of starting with the biggest MCB next to the isolator and going down in size often means this does not happen.

I remember changing a distribution unit because of MCB failures with a larger one so we could fit the half width indicator switches wired back to the control room to alert the operator of a MCB failure and yes you guessed it after that was done they no longer tripped.

We would never consider fitting a resistor start without cooling vents yet often there is no cooling vents in a distribution unit which must generate a lot of heat.

So do you think the design of most of the distribution units is flawed and should manufacturers offer a vented version to address this problem?

 

[PrenticeBoyofDerry]

I would like to know if there is any factual evidence that supports the manufacturers recommendations.
If so are we going to end up with 20 way CUs for 12 circuit split?
Or are we to take in to consideration, the fitting location of the MCB to neighbouring MCBs to avoid or minimise heat dissipation?

 

[ban-all-sheds]

Instead of breakers, what about CTs on each circuit, and monitoring/control logic operating latching contactors?

 

[bernardgreen]

Instead of breakers, what about CTs on each circuit, and monitoring/control logic operating latching contactors?

The job's worth people at Health and Safety get worried when safety depends on "computor" type equipment.

That said I agree that CTs and threshold detectors are a far better way of getting accurate and consistant over load protection. And without any significant heat being generated.

 

[westie101]

The job's worth people at Health and Safety get worried when safety depends on "computor" type equipment.

I doubt they would, all our modern protection is CTs and "computer" derived devices

 

[JohnW2]

Very odd. Their catalogues show MCB's shoulder to shoulder.

They do, as do virtaully all other manufacturer's catalogues and as one would see in most of the millions of CUs/DBs in service. However, of course, the vast majority of those MCBs will not be operating for long periods at close to their rated currents (In).

Wonder what other manufacturer's advice is?

Indeed. As you will have seen from what I quoted, Wylex say "In common with other manufacturers, we recommend..."

Kind Regards, John.

 

[JohnW2]

The ambient temperature around an MCB will affect the tripping point of the thermal trip. It would be interesting to know exactly how much the ambient affects the tripping point. Do the manufacturers publish this information in detail at various ambient temperatures ?.

Such data must exist, but I haven't seen it published in detail. However, the Wylex info includes this limited information:

Wylex MCBs are calibrated to meet the requirements of BS EN 60898,
30°C Ref Calibration Temperature.
At other temperatures the following rating factors should be used:
At 60°C 0.9 At 20°C 1.0 At 0°C 1.1

The manufacturer's use of "diversity" may mean that if you expect a maximum continuous load ( for an hour or more ) of say 20 amps on a circuit then use a 32 amp MCB and size the cable for 32 amps or more if the MCBs are packed together in the CU,

That's roughly the same as the interpretation I suggested. If that's what they mean, I don't see why they decided to use the word "diversity", given that they had used "derating factor" (which seems more appropriate) earlier in the paragraph. Has anyone else got any feelings about interpretation?

Kind Regards, John

 

[securespark]

Wonder what other manufacturer's advice is?

Indeed. As you will have seen from what I quoted, Wylex say "In common with other manufacturers, we recommend..."

Yup, I caught that bit and immediately thought that I could not recall such advice from other manufacturers...

 

[JohnW2]

Wonder what other manufacturer's advice is?

Indeed. As you will have seen from what I quoted, Wylex say "In common with other manufacturers, we recommend..."

Yup, I caught that bit and immediately thought that I could not recall such advice from other manufacturers...

The MK Sentry blurb does not appear to say anything about this. As a passing aside, I noticed the following (from MK), which seems pretty odd to me...

Service short circuit capacity Ics: 6000A
When backed up by a BS 1361, 100A fuse, then the breaking capacity of the MCB is increased to 16,000A

How on earth does the presence of some other upstream OPD affect the breaking capacity of an MCB? Surely it can either break a 16 kA current or it can't?

Kind Regards, John

 

[ban-all-sheds]

Maybe they mean you can then use them on a supply with a 16kA PFC?

 

[JohnW2]

Maybe they mean you can then use them on a supply with a 16kA PFC?

Maybe they do mean that, but (as you saw) what they wrote was "the breaking capacity of the MCB is increased to 16,000A" ... which seems very clear to me, even if a seemingly rather extraordinary statement!

Kind Regards, John

 

[Spark123]

It is the maximum breaking capacity when backed up by the 100A BS1361 fuse, it will take into account the maximum energy let through of the BS1361 fuse (which will probably also operate as well as the MCB if a 6 to 16kA fault occurs). The MCB will probably also be trash with this sort of fault level. It is unlikely, given cable impedances in households that these sort of fault levels will occur that often.