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Posted By Topic: SERF interchangeability with plug in MCBs

OctaneOutlaw
Mar 29 2019 19:03

I'm just trying to understand 2.5.2 note 3

Is it basically saying that if you pull a SERF out that's say a 20A and put a plug in MCB in that the MCB inserted should be rated at less than 80% of what the SERF was

Eg 20A SERF 20*0.8=16 16A plug in MCB

Or should you be reworking out the CCC from scratch then rating the MCB at 80% of the value given?

Any info or theory behind this would be appreciated



   

pluto
Mar 30 2019 17:17

Some additional info in Clause B3 of AS/NZS 3000:2007.

You also need to be able to understand figure B2 to get the full story.
   

AlecK
Apr 01 2019 08:51

If you look again at Note 3; you'll find it says absolutely nothing about the rating of the SERF being replaced. It's only about the relationship between the rating of the new plug-in mcb and the CCC of the conductor.

So yes, you have to assess the CCC of the existing conductor(s). Not forgetting that it used to be common to commence a socket circuit in larger cable, then use smaller for continuing on to the second socket; so you have to check the entire circuit and not judge CCC solely on the size & type of conductor at switchboard.

Whenever we choose overload protection; 2.5.3 applies; so the rating of the protection has to satisfy BOTH equations given in 2.5.3.1.
Equation 2.1 is common sense: MD must not exceed rating, and rating must not exceed CCC. Any circuit breaker will satisfy equation 2.2; however a (HRC) fuse needs to be no more than 0.9 x CCC. A SERF would be much worse than that (they can hold a 100 % overload for more than 4 hours - depending on the cable type, damage could have resulted by then).

But having made the change, consider what happens if someone else swaps out your carefully-selected mcb for another SERF - they're likely to go size-for-size; and end up using a SERF that's got too high a rating.

What the Note is telling you is not to use the biggest mcb you can fit (100 % CCC); but instead to de-rate your protection to 80 % of CCC. This only applies to plug-ins; if you use fixed-wired mcbs you can go to 100 % CCC

   

OctaneOutlaw
Apr 01 2019 12:30

Awesome, thank you both, I'll have a thorough read through that

So basically it's only something to consider with a base that accommodates a SERF being reinsert but if changing the base to one that only supports plug in MCBs it's not going to be an issue?
   

AlecK
Apr 01 2019 12:36

I suspect it was written in days of interchangeable bases and so perhaps less applicable for the "safety" bases we use now. It's only a Note; so it's guidance rather than requirement. While can be seen as good practice, can't be enforced.
   

Someone
Apr 01 2019 18:46

Quote: So yes, you have to assess the CCC of the existing conductor(s). Not forgetting that it used to be common to commence a socket circuit in larger cable, then use smaller for continuing on to the second socket; so you have to check the entire circuit and not judge CCC solely on the size & type of conductor at switchboard.

It's still not hugely uncommon. One group of engineers we work with quite regularly spec long socket circuits as a 4mm feed reducing to 2.5 at the first outlet.
   

pluto
Apr 02 2019 07:41

The correct way to consider what is required is to plot the fusing current and time for each device (MCB or Fuse) on a common axis to see if they are the same, and also get the maximum value and time. This is the value you needed for cable being protected

When you do so you find the factor of 0.9 is needed so the operating points of each device will track each other so the maximum current value for both devices will be very close to each other.

Unfortunely, most device makers use different scales, so a direct comparision is not possible and you have to draw in one the device curve on the other device operating data sheet to get the maximum value.

Some fusing/MCD sizing programmes are now able to do this plotting easily.