EESS / Battery systems Iz of cable

Question

Question I'm embarrassed to ask, you know the old "I should know that......" kinda thing. And kick myself when its explained...

But regarding EESS / Battery systems. where the current capacity of the cable and assemblies must be equal to all loads, for example 16A MCB plus 16A inverter means 32A Iz.

Maybe im being dumb, but why? Surely if inverter is running at full whack of 16A, current cant flow in both directions?

A student asked me this week and I was a little stumped, which as a trainer is somewhat embarrassing!

gkenyon · Accepted Answer

Leccylee said: But regarding EESS / Battery systems. where the current capacity of the cable and assemblies must be equal to all loads, for example 16A MCB plus 16A inverter means 32A Iz.

Maybe im being dumb, but why? Surely if inverter is running at full whack of 16A, current cant flow in both directions? A student asked me this week and I was a little stumped, which as a trainer is somewhat embarrassing! 
 Until very recently (i.e. if the course and exam is based on the 1st and 2nd Editions of the IET CoP for EESS) the thought process was that, as the battery is both a load and a source, 551.7.2 (i) to (iv) applied. It's very 'black and white' as you say, but that's what BS 7671 says. 
 If the course exam is based on 2nd Ed of the CoP, that is what you must teach in order that the learners can pass the exams. This has the unfortunate side-effect that the cable often has to have at least twice the current-carrying capacity it needs to, as you've observed. 
 HOWEVER, In the last few weeks, the 3rd Edition of the IET CoP for EESS has come out. This has a different view of 551.7.2 (i) to (iv), and says that provided there are no other loads on the circuit, you size to I Z &ge; {largest of I n + I g } where 
 I Z is the current-carrying capacity of the cable in the installation conditions 
 I n is the nominal rating of the overcurrent protective device in the CU into which the generator circuit connects 
 I g is the nominal output current rating of the generator (inverter). 
 
 The guidance on what is now 551.7.2 (v) regarding the current rating of consumer units has also changed a little from 2nd to 3rd Ed of the CoP. . The 3rd Ed now contains 3 different approaches as to how to meet the requirement of 551.7.2 (v).

gkenyon · Answer

Leccylee said: But can you explain a little more WHY in versions 1 and 2, it needs to be double the size etc. As I see it a battery system is either "charging" or "discharging", so how can it do both at same time and need to account for both? My only answer thus far to students it must be due to "heating effect", but hate sounding so unsure to class as to "why" 
 The indents (i) to (iv) in 551.7.2 are actually intended if someone wanted to connect a generator in a circuit that does have other loads, and is really not intended to consider the case that a battery storage system is used. There is really nothing more to explain than the technical committee responsible for the IET CoP for EESS took the the stance that BS 7671 had to be complied with verbatim for the first two Editions, and some technical discussions took a more pragmatic view of Regulation 551.7.2 for the 3rd Edition - it's that simple.

EESS / Battery systems Iz of cable

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