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Posted By Topic: Table I1 of 3000 - Imperial Cable Ratings

bjem85
Aug 14 2018 17:49

I have a job I am sizing up where I have a 7/.029 white TRS cable being protected by a 10 amp plug-in circuit breaker. I wish to replace the old ceramic base with a safety base, and I have told the client this 10 amp rating appears somewhat conservative (assuming there is no other reason to limit it such as insulation, other wiring in circuit, or cable temperature ratings), and could be uprated to 16 amp.

What I am a little confused about is that Table I1 of 3000 has different allowable ratings for circuit breakers and plug-in circuit breakers. Is there any particular reason why plug-in circuit breakers are down-rated by a step or two relative to permanently wired circuit breakers?
   

pluto
Aug 14 2018 19:51

You will note that the conductor temperature maximum in the righthand column only has a max conductor temperature of 60 dec C because of the rubber insulation used.

Also note the fuse is an semi-inclosed rewirable fuses in which the actualopewrating point is not actually known and a good safety margin has been allowed for.
   

bjem85
Aug 14 2018 20:20

Thanks Pluto, I don\'t have any problem with why the ratings exist, it\'s the difference between ratings of CB and plug-in CB.

For example, if you look on page 541 of 3000:2018, you can see for 7/.029 when partially surrounded that the protective device rating for V60 is either 16A in the \"CB\" column, or 12A in the \"Fuse or Plug-in CB\" column. I am confused about why the current ratings would be different, when it doesn\'t appear to be anything to do with the cable itself. Are the tripping curves on plug-in circuit breakers different?
   

pluto
Aug 14 2018 20:44

When the drafting of AS/NZS 3000:2018 was being done we had a feeling that someone would like the theory for the currents differnces explained. The theory is detailed in Appendix B in Section B3 in the AS/NZS 3000:2018 edition only.

The key part is there is 0.9 factor for fuses when compared with a MCB cable current ratings; and then Table I 1 will be rounded down to the next commonly availabe fuse wire.

It is all connected with heating of the conductor when carrying current and the melting point of the cable core insulation.
   

bjem85
Aug 14 2018 21:58

Thanks Pluto, I can see the 0.9 factor coming from B3.2.2.3 for fuses. It doesn\'t say anything about plug-in circuit breakers.

So my question would be why are the two columns in Table I1 labelled \"Fuse or Plug-In CB\" instead of just \"Fuse\"? From what you\'re saying, the derating should only apply to fuses.
   

pluto
Aug 15 2018 07:48

You have found a possible error in AS/NZS 3000:2018 in Appendix B, but I will need to check the time /current values of a typical plug MCB to see if they are the same as for a typical switchboard mounting MCB conforming to AS/NZS 60898.1.

I don\'t have a set data for plug in MCB\'s to check and it may take a few days for me to get some data and be able to give you a full answer.
   

pluto
Aug 15 2018 08:22

I have a typical data sheet for a plug in MCB and they conform to AS/NZS 60898.1 so it appears that the headings columns in table I1 may be incorrect. That will take sometime ro check.
   

bjem85
Aug 15 2018 09:36

Thanks Pluto, I thought it may have been something like that. Hopefully it\'s easy to verify and fix.
   

Andrew
Aug 15 2018 11:17

I think the reason for \"fuse or plug-in CB\" ratings is because the two are interchangeable - if there\'s a plug-in CB there now, anyone could change it for a SERF tomorrow and the cable may no longer be adequately protected if they use the same fuse rating as the old CB. The lower rating ensures that if a CB is replaced by a fuse, the fuse will still protect the cable.
   

bjem85
Aug 15 2018 11:27

Andrew, I take it then if you replace a ceramic fuse base with a safety base there should not be anything to worry about, and no special de-rating for the future possibility of fitting a fuse is required.
   

AlecK
Aug 15 2018 14:36

While I wasn\'t part of the team that wrote that bit, Andrew\'s explanation rings true
   

Someone
Oct 06 2018 16:17

I realise this is a bit old, but I think it should be noted that the source seems to be note 3 in 2.5.2 (both 2007 & 2018):

\"However, because of interchangeability with semi-enclosed rewireable fuse-carriers, such circuit breakers should be rated at not more than 80% of the current-carrying capacity of the protected conductor.\"


Given the \'should\' and it being a note, plus the points above about the new plastic bases not fitting fuses, I think it could generally be ignored.