Max Zs for D type mcb

[streamer]

I have a 40 amp 3 phase motor to install using a star-delta starter for a large fan in a grain dryer. The manufacturer specifies a 50 amp d type breaker. The problem is the max Zs for this breaker is 0.23, the Ze of the installation is 0.19 and the motor is to be situated quite a distance from the origin. Even by over sizing the cable it is impossible to achieve the required Zs. Any suggestions?

Many thanks

 

[telectrix]

only thing i can suggest, is contact manufacturer and see if it's possible to use a C type. otherwise, over to tony

 

[Darkwood]

Use HRC fuses the Zs allowance is comfortably achievable without increasing core size to sillyness size.


PS Welcome to the forum

 

[Bobster]

As dark has already said, use Hrc fuses.

Or fit separate earth leakage protection.

 

[Darkwood]

As dark has already said, use Hrc fuses.

Or fit separate earth leakage protection.

You don't design a circuit that fails zs values then add rcd protection to get it to comply, you design it to comply from the off and rcd covering a motor isn't really a good idea....

 

[Bobster]

Not a good idea, and I wouldn't design something like that myself.

It is however an option.

 

[Engineer54]

Not a good idea, and I wouldn't design something like that myself.

It is however an option.

Not to an experienced qualified Electrician it's not!! Now if you're talking about a electrical trainee or a bodgit joe, where anything goes, you may well have a point!! But you DON'T design a circuit that doesn't or can't comply with required Zs values for the OCPD, ...ever!!!

 

[Darkwood]

@Rob2 -Will have to disagree any new circuit should be designed for short circuit protection ie meeting zs requirements you shouldn't be designing circuits that can't meet zs requirements then adding rcd protection to comply, in the OP situe a correctly designed install will not have Zs issues as i have give a simple solution and going down the route of poor design then adding rcd to comply is a bad option with the OP's motor circuits...

Its akin to designing a pelican lights green man timer to lasting 3 seconds and making sure everyone that crosses wears reflectives for when the timer runs out mid crossing.

The regulation is there for where alternatives to meet Zs can't be met due to limitations like the nature of the supply but regarding the OP's case a correctly designed install will not have Zs issues so no reason to consider that approach so poor advice as it shouldn't be considered an option.

 

[Darkwood]

Nice of the OP to thank us ..... saw him on line on this thread for my first few replies ... i know its kinda charity advice but appreciation is always welcome!

 

[Bobster]

I think my point came across wrong. Of course earth leakage protection is additional.

Ive been taught and practice the same philosophy.

I will add I have used EL protection on circuits that have been damaged, as an interim measure so they can maintain in service.

 

[streamer]

Many thanks to all who posted, it is very much appreciated!

 

[telectrix]

you can also use the thanks button

 

[streamer]

Sorry guys, I didn't mean to show a lack of appreciation. I'm just finding my way round the forum and didn't see the thanks button. I hope I haven't upset everyone with my very first post! As I said I do very much appreciate your input, and look forward to taking part in further discussions.

 

[telectrix]

no worries, mate.