water heater

[Banburyman]

1.
first problem- water heater 8kW, separate circuit. Do I need put a switch between Fuse board and water heater?


2.
another problem - 14kW water heater 3 phase,

I = P/Ö3 *400 = 21A

this is 21A, how to find right fuse? in 17Th edition there are values for U=230V,where to find max earth fault loop impedance (Zs) for circuit breakers with U=400V

 

[Guest123]

1.
first problem- water heater 8kW, separate circuit. Do I need put a switch between Fuse board and water heater?

2.
another problem - 14kW water heater 3 phase,

I = P/Ö3 *400 = 21A

this is 21A, how to find right fuse? in 17Th edition there are values for U=230V,where to find max earth fault loop impedance (Zs) for circuit breakers with U=400V.

1. Yes, you will need a local form of isolation for the water heater, i.e somewhere close to it.

2. Max Zs values in 7671 apply to three phase breakers also as the (Uo) i.e voltage between phase and earth, is 230V on both single and three phase.

 

[Banburyman]

I dont know Ze value. According instruction they recommend 25A MBC type C where max Zs = 0.92
Lenght of circuit is 20 meters,
Can I use 25 RCBO type C instead ( 100mA) ?
cable diameter 6mm2 line, and 4 mm2 CPC, going in suspended cable trunking with another the same cable, It is correct?

 

[Guest123]

Hi.

If you feel that the Zs may be too high for the C type MCB then you have two options available -

1. Increase the size of the CPC that you are going to install (if you're wiring in singles then not a problem)

2. Offer additonal protection by using an RCD. RCBO's are only available in single pole 30mA guise and this may or may not cause nuisance tripping of the water heater so maybe a 100mA RCD and 25A MCB in a small enclosure just for the water heater, assuming you are referring to the single phase heater???


To design correctly you would need to find out the Ze and carry out the cable calcs neccessary.

HTH.

 

[Banburyman]

referring to the three phase heater

customer wants one single heater (8 kW), and two 14 kW, mess in Cu, no Periodic Report,
What I saw today, this is CU (incoming tail is a SWA armored), I tried to discribe this CU on my drawing. I have no experience on domestic installation, but something like this I saw first time.
What is protected by RCD?