To be competitive, some electrical contractors often supply low-voltage (LV) service entrance switchboards and switchgear that meet the minimum safety requirements of the National Electrical Code (NEC). The Code stipulates that a ground-fault relay must be installed on the service entrance for services with a rating higher than 1,000A. This relay shall be set for no more than 1,200A, and it shall trip the service entrance breaker.
As related to safety, this is a great improvement over earlier requirements of the Code, which did not call for any ground-fault relay. However, it must be noted that this level of ground sensing and time delay protection arrangement provides limited coordination with any possible downstream protective devices. As a result, when the main service entrance device trips the entire building suffers a blackout.
So what can you do to improve the overall protection and coordination of the power delivery system? Let’s take a look at some other options you have to improve system performance and better serve your customer.
As per Code requirements, all 3-phase, 4-wire circuits, where the neutral is distributed and used for the load(s), must be solidly grounded. As a matter of habit, many designers and installers also solidly ground 3-phase, 3-wire systems. In such systems, the ground-fault sensing relays, which are installed at various levels throughout the distribution network, are time-current coordinated.
Where there are multiple levels (i.e., zones) in the power system, there is a need for coordination of the zones so that, whenever possible, the higher levels are unaffected by downstream faults. The branch circuits are like tree branches, and all of the relays will “see” the fault current in a particular branch when the fault is downstream.
Typically, you achieve coordination by setting protective relay time-delays progressively higher, with upstream relays set to maximum delays so as to prevent nuisance tripping of the breaker. However, should a fault develop at a high level and require the time delay to expire before clearing the fault, this set-up can cause unnecessary damage to downstream equipment.
This design approach raises the cost of the overall system due to the large number of ground-fault sensing relays required. It also brings about longer relay operating times due to the increased time delay settings associated with the time coordination of the multiple protective devices on the system. The source relay has the longest delay in order to prevent the main relay from tripping first.
Fig. 1 shows a time-coordinated system of ground-fault relays in a low-voltage distribution arrangement. Its relay scheme operates as follows:
- GFP-1 responds after a time delay of 12 cycles to any ground fault that hasn’t been cleared by GFP-2 or GFP-3.
- GFP-2 responds after a delay of 6 cycles to any fault that hasn’t been cleared by GFP-3.
- GFP-3 responds instantaneously to any fault on the branch circuit it protects.
In this scheme, the time delays necessary for conventional time-current coordination may compromise equipment protection. For example, in the event of an arcing ground fault in these solidly grounded systems, the circuit is tripped with delay, resulting in potentially unacceptable damage.
An alternative arrangement is to use zone-selective instantaneous protection (ZSIP), where the ground-fault sensing relays are all set for instantaneous trip protection and the downstream relay will signal the upstream relay in the upper zone that it will clear the fault and block the upstream relay from tripping. This scheme provides coordination with instantaneous clearance of arcing faults, thus preventing major damage at all levels in the system.
Fig. 2 shows a distribution system with ZSIP. This type of system works as follows:
- GFP-1 responds instantaneously to ground faults on the line side of GFP-2.
- GFP-2 responds instantaneously to faults on the load side of its location to the line side of GFP-3 and sends a restraining signal to GFP-1.
- GFP-3 responds instantaneously to faults on its load side and sends a restraining signal to GFP-2.
On 3-phase, 3-wire low-voltage systems where solid grounding is specified, high-resistance grounding may be offered to improve power continuity, eliminate damage due to ground faults, improve coordination and selectivity, and provide a system that enables safe operation and low maintenance costs.
As a designer or installer you can take a proactive approach with your customers and recommend that better coordination and protection of electrical equipment can result in less equipment damage and fewer widespread outages. The choice is yours.
Murray is consultant and technical support manager for IPC Resistors, Inc., in Mississauga, Ontario, Canada.