Figure 14.10 LV Circuit breaker calculator

The following is a description of the input/output fields:

Electrode configuration
There are five different electrode configurations, and this field is a drop-down list giving choices as follows:

• VCB: Vertical conductors/electrodes inside a metal box/enclosure.
• VCBB: Vertical conductors/electrodes terminated in an insulating barrier inside a metal box/enclosure.
• HCB: Horizontal conductors/electrodes inside a metal box/enclosure.
• VOA: Vertical conductors/electrodes in open air.
• HOA: Horizontal conductors/electrodes in open air.

Chapter 8: Data Collection describes which electrode configuration to use as the choice will determine not only the arcing current but also the incident energy and arc flash boundaries.

System voltage
The three-phase line to line voltage in volts.

Prospective short circuit current (kA)
This is the user’s estimate of prospective short circuit current in kA whilst seeking to consider system variations. The user is advised to enter both upper and lower limits of prospective short circuit current to determine which produces the worst-case incident energy. A lower fault current can result in a higher value of incident energy.

Conductor gap distance (mm)
Enter the gap between conductors at the equipment. This may need some engineering judgement as described in Chapter 8: Data Collection.

Working Distance (mm)
This is the distance between the potential arc source (not just the front of the panel) and the face and chest of the worker.

Circuit breaker rating (amperes)
This is the overcurrent rating of the circuit breaker or long-time current setting as shown in Figure 14.9 earlier, (not the frame size or rating).

Instantaneous pickup level
This is the instantaneous setting which is usually between 2 and 15 times the overcurrent circuit breaker rating for integral electronic trip units. This may be a fixed value on older thermal magnetic trips and miniature circuit breakers.

Instantaneous pickup switched on?
Verify that the instantaneous section or setting is switched to on. The calculator has a YES/NO dropdown menu to state that the instantaneous element is switched on by selecting YES. Select NO if the instantaneous setting is switched off.

Make and type
Pick from a dropdown list of circuit breaker make and types. This is the first check to see if the circuit breaker is listed in a library of circuit breakers in use in Europe. For the circuit breakers to be listed, they will pass the criteria given later under circuit breaker listings. You can also select “(Valid) Make/Type Not Listed” in which case the circuit breaker meets the criteria but is not listed or “(Invalid) Make/Type Not Listed” and in this case the circuit breaker will always default to two seconds disconnection time.

Model
Pick from a dropdown list of circuit breaker models.

Protection
Pick from a dropdown list of circuit breaker integral protection unit types if fitted.

Frame Size
Pick from a dropdown list of circuit breaker frame sizes if given.

Incident energy (cal/cm2)
The calculator will produce an incident energy level based upon either, instantaneous operation of the circuit breaker or a two second maximum disconnection time.

Arc flash boundary (mm)
The arc flash boundary will also be stated based upon either, instantaneous operation of the circuit breaker or a two second maximum disconnection time.