Placement of mcb's in my CU

[SimonH2]

But it makes you wonder why when you buy fully loaded boards, that the MCBs that are pre-installed. From my experience start with the higher amp device at the isolator. Whether it be the main or RCD.

From the "we've always done it that way" school of life.

 

[ban-all-sheds]

http://www.unixsage.com/index.php?option=com_content&task=view&id=44&Itemid=64

 

[JohnW2]

I would say, that method of instalment could well make sense. But it makes you wonder why when you buy fully loaded boards, that the MCBs that are pre-installed. From my experience start with the higher amp device at the isolator. Whether it be the main or RCD.

I think the answer to that is simple enough. Whatever was the original reason, we are all agreed that there is very well established 'traditional wisdom' about this and a strong notion that the traditional method is 'good/best practice'. I've certainly always followed that 'traditional wisdom', and I imagine that most other people her have done so as well. The MIs of CUs may well say the same thing.

However, as I said, it does sometimes pay to stand back and ask whether these very-long-standing traditions actually do make sense.

Kind Regards, John.

 

[DetlefSchmitz]

In fact, I would have thought that a more relevant (than voltage drop) consideration would be that of heat generated within the MCBs when heavily loaded - and that would best be addressed by advising that one should not put high-rated MCBs next to one another (but, rather, should put low-rated ones in between them), wouldn't it?

Kind Regards, John.

I think this is a big assumption. I have always supposed that the heat generated is that which is designed to operate the trip mechanism. If so, then the higher rated MCBs would have a lower resistance and end up reaching a similar temperature at full load.

 

[JohnW2]

I think this is a big assumption. I have always supposed that the heat generated is that which is designed to operate the trip mechanism. If so, then the higher rated MCBs would have a lower resistance and end up reaching a similar temperature at full load.

Yes, that's a good point. Perhaps more to the point is that the sort of circuits fed from the highest rated MCBs tend to be ones (showers, cookers etc.) that will quite often be almost fully loaded, whereas 'full loading' is much less common for, say, socket and lighting circuits. A 6A lighting circuit MCB between the shower and cooker ones might therefore make some sense.

Anyway, as I'm sure you understand, I was not suggesting that we should deliberately interspace high rated MCBs with low rated ones - in practice, I don't think the ordering is of any significant importance. I was merely pointing out that not only does there seem to be little engineering basis for the 'traditional wisdom' but it's even possible to argue that the 'traditional' layout is not always 'ideal'.

Kind Regards,
John

 

[DetlefSchmitz]

I think this is a big assumption. I have always supposed that the heat generated is that which is designed to operate the trip mechanism. If so, then the higher rated MCBs would have a lower resistance and end up reaching a similar temperature at full load.

Yes, that's a good point. Perhaps more to the point is that the sort of circuits fed from the highest rated MCBs tend to be ones (showers, cookers etc.) that will quite often be almost fully loaded, whereas 'full loading' is much less common for, say, socket and lighting circuits. A 6A lighting circuit MCB between the shower and cooker ones might therefore make some sense.

Kind Regards,
John

Since you mention it, it would be equally reasonable to say that a shower is used only for a few minutes a day, and it would be common in some locations for one 6A mcb to carry a full load all day in certain types of premises (I'm sitting in such a place at the moment). Plus the design is such that it may take up to an hour to reach critical temperature. So I don't think any particular arrangement is best when it comes to temperature effects.

 

[JohnW2]

So I don't think any particular arrangement is best when it comes to temperature effects.

You're probably right and, as I said, I find it hard to believe that there is anything much to choose between different arrangements in relation to any considerations - and I would certainly challenge the basis for any suggestion that any particular arrangement was 'not good practice'.

Kind Regards, John.

 

[JohnW2]

As a matter of interest, I note that Wylex say (and others probably say much the same):

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).

Kind Regards, John.

 

[DetlefSchmitz]

I did find that mildly surprising.