UPS protection and distribution. 1

[squeeky]

At an installation there is a 400v, 50Hz, 3 ph,4wire, DB with a Fault Level of 35kA. It is fed from a 1MVA, 11kV to 400v, DYn11 trf. This DB feeds a 40kVA, three phase, UPS device. In the Main DB the breaker feeding the UPS is a 125A thermal magnetic MCB set at the correct levels. No earth connected.

UPS feeds out with a 4 wire system but is not earthed. It feeds on a 100A breaker to a SDB. 25mm sq, 4 core, SWA, ECC, PVC, PVC. The SDB then feeds 9 Emergency Lighting DBs with 3 pole MCBs. Still no earth. These DBs feed a 3 ph, 4 wire lighting circuits from a 32A, 3 pole breaker. The cable is a 16mm sq four core. It feeds lights in a single circuit with different phased tapped of at each light (220v single phase) to balance the load. Question 1. What should the correct protection method be? 4 wire or 3 wire / pole breaker?

Question 2. What happens when luminaires start to fail and they are all happen to be on one phase but the breaker does not trip. The voltage on some phases will go up so why no earth?

Quesstion 3. With the first ground fault there is no path so no current. When the second ground fault happens will the 32A MCB trip? I say it depends where the fault is.

 

[sibeen]

I'm going to assume that the star point of the transformer is earthed, is that correct?

Assuming that, then:

1. 3 Pole protection is required.

2. Changing load balance will not effect the voltage balance.

3. There is a path for fault current to flow under the first fault to ground.

Saying that, I have worked with isolated UPS systems in the past but they certainly didn't have this configuration. It seems very strange.

 

[squeeky]

Dear Sibeen, Sorry I travel ad have been away in Swaziland where this problem exists. The transformer feeds into the UPS. Yes the star point is earthed. The UPS runs a system where it charges a DC bus which is then chopped up into three phase AC so the primary and secondary sides of the UPS are seperated by electronics. Each lighting circuit is fed on a three phase 16A triple pole circuit breaker.

If we lose a phase load the phase voltage will shift on the remaining two phases if it is not earthed. The problem is that the customer wants the system isolated so that it does not trip with an earth fault. There are two issues. Firstly is that with one earth fault it will not trip, but if comeone become the second earth path, they will be electricuted. Secondly if we are running daisy chained with lights in parallel on say red phase and the point of coupling to the next light goes open circuit, the remaining lights are pulled off the circuit so the voltage to the neutral is pulled up due to the unballanced load.

I'm looking for someone who can supply a standard or regulation number for this type of installation. Thanks for the come back.

 

[sibeen]

squeeky, I must state that I really don't understand.

As the star point of the transformer is earthed, then the neutral is earth referenced and tied to that earth. This means that the phase voltages also have an earth reference and in any fault to earth a substantial current will flow.

Also, as the neutral is tied to the earth then a current phase imbalance is not going to be moving the neutral point, and the voltage levels from each phase to the neutral will remain the same.

If you require an isolated UPS, and it is quite common in oil refineries etc, then you need to be feeding the input of the UPS from a delta secondary. This way the output neutral of the UPS will have no earth reference.

 

[sibeen]

Ooops, you can also do it with a star secondary, but not if you go and then earth the star point.

 

[squeeky]

Hi Sibeen, My English! The inverter has a 400v, 4 wire primary, from a star transformer, 11 000v to 400v, 2 000kVA, DYn11. The primary of the inverter / UPS is indipendant to the output. The inverter runs all the time. It converts the 400v AC to DC on a bus which charges teh Battery backup. This bus supplies the inverter side and gives 400v, 230v to neutral on the output. The output is star and has a star point so the phase neutral is 230V AC. This star point is not earthed.

I will be back in South Africa and the office this week and will try to get a sketch to you. Thanks for the assistance so far.

 

[ScottyUK]

Most codes require that a source serving loads connected line-neutral has the neutral connected to earth.

 

[sabersteg]

Dear All,
Just to clarify taht there is many distribution system in standard: TT, TN, TNS,IT,... depending on neutral earthing. In your case you have an IT system so the neutral is isolated from earth. Also we can have the neutral from transformer linked to earth but not distributed.
Anyway, for the UPS we have rectifer and inverter: for the inverter Dc voltage is inverted to AC voltage 230VAc without neutral to earth..
Kind regards