A time current curve also known as a TCC plots the interrupting time of a protective device based on the current level. These curves are used to determine coordination between protective devices such as fuses, breakers & relays. The curves also show non-protective equipment such as motors, cables and transformers as the protective equipment must be set to coordinate with motor starting and protect the cables and transformer so the breaker trips before current reaches levels of the equipment’s damage curve.
A general trend for proper coordination involves the lowest protective level being the furthest left and down of equipment on the TCC, for example a feeder breaker should be set to trip at lower levels than a main breaker to minimize loss of equipment in the scenario of a fault. Another consideration associated with coordination and TCCs is Arc Flash, as TCCs show interrupting times at various currents it makes sense that there is a relationship between interrupting times and the Arc Flash Incident Energy.
Given proper coordination in the ideal situation, only the faulted circuit will experience an outage.
Maximizing protection, while also optimizing selective coordination, is not always possible due to the limitations of current protective equipment and, in some cases, the limits of protective technology. In recommending improvements to the protective scheme, it is necessary at times to improve protection by sacrificing some coordination and at other times to sacrifice some protection to gain improved coordination.
Limit your TCCs to 3 or maximum 4 Protection Devices (PDs)
Make sure you are overlapping your downstream PDs to the upstream ones e.g if you split your TCC into two new TCCs you would want to show the LVCB# MB 117-088-002 in both the new TCCs.
Try to utilize combine curve option (might not be available on all softwares, but I have seen it in ETAP) this shall save you some space.
Also, the following could be beneficial to know:
You might need to show the minimum fault currents, transformer inrush current and cables thermal damage curves (if any) on your TCC. This to ensure an adequate zone protection (settings shall be sensitive enough for minimum fault currents and cables damage curves while allowing for transformer inrush current to pass with no unnecessary tripping).
Basically split your Protective Device into several TCC’s, so you can analyze and set it selectively in a more efficient manner.
