Hello everybody. I’m planning an 2

[Carlos Melim]

Hello everybody.

I’m planning an IT earthing system to feed some emergency pumps within an existing building.
The existing electrical installation is on a TT scheme.

There will be, of course, a specific LV/LV transformer with a Permanent insulation monitor.
I will install an equipotential cable between the transformer, the drives and the pumps.
But I can’t isolate all the metallic elements from the building protection earth.

If there’s a phase to earth fault, 1st fault, the pumps will still work but the building PE cables voltage level will be the one of the IT system faulty phase.

Will there be damages to other single phase electrical equipments of the building?

I think the potential difference between the phases of the TT system and the PE cable will not to result in any current.

Should I rigidly connect the equipotential cable to the PE of the building?

I am looking forward to hearing your advices.

Best regards.

Carlos Melim

 

[waross]

If there’s a phase to earth fault, 1st fault, the pumps will still work but the building PE cables voltage level will be the one of the IT system faulty phase.

I wouldn't put it that way.
That statement may be misleading.
When one phase is faulted to ground, that phase assumes ground potential and the other two phases assume line to line voltage to ground.
Normally on an isolated ground system the phase voltages are at 1/1.73 line to line voltage to ground.
This is due to cable charging capacitance and resistance leakage current.
The codes that I work under require ALL exposed metal surfaces to be bonded to earth.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Carlos Melim]

This is what I meant:

 

[waross]

That installation is contrary to the codes that I work under.
"The neutral shall be grounded if by so doing the phase voltage to ground will not exceed 150 Volts."
OR
"Where line to neutral loads are served, the neutral shall be grounded."
Apart from that it will work subject to:
On a 400 Volt circuit the phase conductors normally operate at 230 Volts to ground.
The neutral is at ground potential.
In the event of a ground fault, the unfaulted phases will rise to 400 Volts to ground.
The neutral will rise to 230 Volts to ground.
Line to neutral connected equipment will still see 230 Volts.
Personally I would hesitate to trade the ability to operate with one phase grounded for the possibility of widespread damage due to an arcing ground fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricuwe]

Your post is somehow ambiguous. Are you installing the IT-system for the complete building or just for the pumps ?

The later case would be the usual one.

IT systems are used for installations where you do not want to stop the process due to a single isolation faults. Some examples are factories processing molten metal or molten glass. For several reasons you usually do not want to extend this mode of operation to the whole building.

On such a dedicated IT system it is quite uncommon to have single phase loads, however this would not constitute a general problem. When choosing your equipment, you have to ensure that the isolation is specified for this. Especially in drives you might need to restrict your range of choices or to disconnect some overvoltage protection. Consult the manual for this.

In case of a fault to ground (motor frame, enclosure or similar), there will be a ground fault current limited by the capacitances of the system. Properly designed the PE conductors will carry this current without raise of the local ground potential to a dangerous level.

 

[Carlos Melim]

Hello electricuwe

You are correct. IT system is only for the pumps.

I understand the emergency equipments must comply with IT requirements. For example, if they are single phase, they must be suitable to a 400VAC voltage between phase and earth. The pumps will be delta connected but the control systems can be single-phase.

My concern is the rest of the building electrical equipments.

If a 1st fault appears on the IT system, the PE will raise to 400VAC. But, for the building other electrical equipments, that voltage will not be a concern because it comes from the IT isolation transformer, not from the grid that supplies the building. A voltage that results in no current.
Am I right?

 

[electricuwe]

Carlos,

yes, single phase equipment in the IT system has to be isolated for 400 V against ground. Single phase load might be control. But at least in Europe control equipment in three-phase installations is usually supplied via a control transformer (primary 400 V, connected between phases, secondary 230 V).

But in all cases, both PE of the remaining building as well as PE of the IT-system will stay close close true ground. The impedance Z (quite high in this approach) will limit the current in the PE connection and hence also limit the raise of the local potential of PE.

 

[che12345]

Dear Mr. Carlos Melim,

1. I am of the opinion that the proposed IT-system is fine, if you limit it to the pumps only; which your intention is (not to trip off) at the [first] earth-fault.
2. The proposed scheme shows that a [4-pole] breaker was used for the incoming and the three-phase with neutral load. A [2-pole] breaker (which also breaks) the neutral, for the single-phase load. The local PE is to be connected to the main equipotential link. Fine.
3. Any earth-fault between any [Line and earth] will be detected by PIM . Fine.
4. I am concern that a [short between the Neural and earth] will NOT be detected by the PIM. Although a short between the Neutral and earth will [NOT trip the system], but certainly NOT desirable. As it is not detected by the PIM, the fault would persists unnoticed. The system would be changed from IT to TT , which is contradictory to your intention.
Am I wrong some where ? Why no one has raised this concern?
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che; You may be correct. However the sketch does show the PIM connected to the neutral.

Why no one has raised this concern?

Probably because most of us work under codes that prohibit line to neutral loads on most ungrounded systems.

Bill
--------------------
"Why not the best?"
Jimmy Carter