Technical Overview

This section contains basic technical information pertaining to Power-Style QED-6 switchboards, MasterPact circuit breakers, PowerPact circuit breakers, and MicroLogic trip units.

QED-6 Switchboard Ratings

Switchboard Ratings

Application Voltage Systems	Ampacities	Interrupting Rating
120 Vac–600 Vac maximum 1Ø3W, 3Ø3W, 3Ø4W 50/60 Hz	800–5000 A (main circuit breaker or main lugs only) 6000 A thru bus	Up to 200 kAIR @ 240 Vac Up to 150 kAIR @ 480 Vac Up to 100 kAIR @ 600 Vac
Equipment short-time rating	800–6000 A	Up to 100 k with MasterPact NW Up to 65 k with MasterPact NT Up to 6 k with PowerPact D

Circuit Breaker Ratings

Circuit breakers are available in various levels of interrupting ratings (AIR) as listed below.

Interrupting Ratings of Circuit Breakers (RMS Symmetrical Amperes)

Circuit Breaker Model	Circuit Breaker Frame Size	Trip Current Range	Circuit Breaker Model No.	Interrupting Rating			Short-time Rating
Circuit Breaker Model	Circuit Breaker Frame Size	Trip Current Range	Circuit Breaker Model No.	240 V	480 V	600 V	@ 480 V
PowerPact H	150	15–150	H-G	65 k	35 k	18 k	N/A N/A N/A
	150	15–150	H-J	100 k	65 k	25 k
	150	15–150	H-L	125 k	100 k	50 k
PowerPact J	250	175–250	J-G	65 k	35 k	18 k	N/A N/A N/A
	250	175–250	J-J	100 k	65 k	25 k
	250	175–250	J-L	125 k	100 k	50 k
PowerPact D	150	60–150	D-N	65 k	35 k	18 k	6 k
	150	60–150	D-H	100 k	65 k	25 k	6 k
	250	100–250	D-N	65 k	35 k	18 k	6 k
	250	100–250	D-H	100 k	65 k	25 k	6 k
	400	160–400	D-N	65 k	35 k	18 k	6 k
	400	160–400	D-H	100 k	65 k	25 k	6 k
	400	160–400	D-L	150 k	100 k	25 k	6 k
	600	240–600	D-N	65 k	35 k	18 k	6 k
	600	240–600	D-H	100 k	65 k	25 k	6 k
	600	240–600	D-L	150 k	100 k	25 k	6 k
MasterPact NT	800	40–800	NT08N	50 k	50 k	35 k	35 k
	800	40–800	NT08H	65 k	50 k	50 k	35 k
	800	40–800	NT08L1	100 k	65 k	N/A	10 k
	800	40–800	NT08L	200 k	100 k	N/A	10 k
	800	40–800	NT08LF	200 k	100 k	N/A	10 k
	1200	240–1200	NT12N	50 k	50 k	35 k	35 k
	1200	240–1200	NT12H	65 k	50 k	50 k	35 k
	1200	240–1200	NT12L1	100 k	65 k	N/A	10 k
	1200	240–1200	NT12L	200 k	100 k	N/A	10 k
	1200	240–1200	NT12LF	200 k	100 k	N/A	10 k
MasterPact NW	800	40–800	NW08N	65 k	65 k	50 k	42 k*
	800	40–800	NW08H	100 k	100 k	85 k	65 k*
	800	40–800	NW08L	200 k	150 k	100 k	30 k*
	800	40–800	NW08LF	200 k	150 k	100 k	22 k
	1200	240–1200	NW12N	65 k	65 k	50 k	42 k
	1200	240–1200	NW12H	100 k	100 k	85 k	65 k
	1200	240–1200	NW12L	200 k	150 k	100 k	30 k
	1200	240–1200	NW12LF	200 k	150 k	100 k	22 k
	1600	320–1600	NW16N	65 k	65 k	50 k	42 k
	1600	320–1500	NW16H	100 k	100 k	85 k	65 k
	1600	320–1600	NW16L	200 k	150 k	100 k	30 k
	1600	320–1600	NW16LF	200 k	150 k	100 k	22 k
	2000	400–2000	NW20N	65 k	65 k	50 k	42 k
	2000	400–2000	NW20H	100 k	100 k	85 k	65 k
	2000	400–2000	NW20L	200 k	150 k	100 k	65 k
	2000	400–2000	NW20LF	200 k	150 k	100 k	22 k
	2500	500–2500	NW25H	100 k	100 k	85 k	65 k
	2500	500–2500	NW25L	200 k	150 k	100 k	65 k
	3000	600–3000	NW30H	100 k	100 k	85 k	65 k
	3000	600–3000	NW30L	200 k	150 k	100 k	65 k
MasterPact NW (31 inches wide)	4000	800–4000	NW40H	100 k	100 k	85 k	85 k
	4000	800–4000	NW40L	200 k	150 k	100 k	100 k
	5000	1000–5000	NW50H	100 k	100 k	85 k	85 k
	5000	1000–5000	NW50L	200 k	150 k	100 k	100 k

For additional short-time ratings, refer to the appropriate circuit breaker catalog sections.

Distribution Systems

This section contains a brief overview of Wye and Delta distribution systems.

Wye Solidly Grounded Systems

A Wye system is the most common type of three-phase distribution system for low voltage switchboards. Wye systems are either three- or four-wire distribution systems that are normally grounded, either in the equipment (see Four-Wire Wye System Fault Detection By Way of Current Relay) or at the transformer source (see Three-Wire Wye Distribution—Solidly Grounded System).

Four-Wire Wye System Fault Detection By Way of Current Relay

Three-Wire Wye Distribution—Solidly Grounded System

When the system is grounded in the equipment and the neutral phase is carried through the equipment, the system is described as a four-wire solidly grounded system with neutral connections available to supply single phase-to-neutral loads, such as lighting loads.

When the system is grounded at the transformer source and no neutral phase is carried through the equipment, the system is described as a three-wire solidly grounded system. No neutral connections are available; all loads must be three-phase (or single “phase-to-phase”) loads.

In some cases, the neutral is delivered to the service entrance where it is bonded to ground similar to the four-wire solidly grounded system.

Delta Grounded and Ungrounded Systems

Formerly common, Delta three-wire systems are rarely used in low voltage distribution systems. Delta three-wire distribution systems can be grounded or ungrounded services. Generally, Delta systems are ungrounded. In some cases, they are grounded on the “corner” of the Delta or some other point. Ungrounded Delta systems do not have a reference point or ground. Corner or Grounded B Phase Delta distribution systems do provide a reference point, but require one phase to be connected to the ground.

Three-Wire Delta Distribution—Ungrounded

Although ungrounded systems have been used for many years, they are not recommended in newly designed low voltage distribution systems. Ungrounded power systems may be unstable. High resistance grounded systems as shown on Wye Systems Configured and Delta Systems Configured are recommended for use in newly designed low voltage distribution systems.

Equipment Ground Fault Protection for Wye Systems

The need for equipment ground fault protection in three-phase solidly grounded Wye systems is due to the possibility of low magnitude equipment ground fault currents. Phase-to-phase faults are of such a large magnitude that overcurrent protection devices (trip units) can operate rapidly and positively to signal the circuit breaker to open. Equipment ground faults can be of a much lower magnitude and require specialized elements in the trip unit for detection.

QED-6 switchboards with MasterPact circuit breakers equipped with MicroLogic trip units have the capability of providing equipment ground fault sensing (alarm with no tripping) and equipment ground fault protection (tripping) on three-phase, three-wire and three-phase, four-wire solidly-grounded systems. MasterPact circuit breakers can be used for overcurrent protection on ungrounded or resistance grounded systems, but are not suitable for equipment ground fault protection on these systems. Equipment ground faults are an inadvertent flow of current between the normal current-carrying conductors and ground. These ground faults usually start at a low level and, if left undetected, escalate causing significant heat and mechanical damage to the electrical system. Equipment ground faults can escalate to a phase-to-phase fault causing major system damage. The equipment ground fault system in the MasterPact circuit breakers monitor the flow of current in the system and detects equipment ground fault currents. The circuit breaker will trip to protect the circuit, or send an alarm through the appropriate interface equipment, depending on the option installed.

MasterPact Circuit Breaker Ground Fault System Sensing Capabilities

The National Electric Code (NEC) requires equipment ground fault protection on solidly grounded Wye electrical services of more than 150 V to phase-to-ground, but not exceeding 600 V phase-to-phase for each service disconnect rated 1000 A or more. This includes service entrance, feeders and building disconnects. The NEC also requires equipment ground fault alarm (no tripping) on emergency systems and allows equipment ground fault alarm on continuous processes, fire pumps, and other circuits that would be more hazardous if stopped without an orderly shutdown.

Equipment Ground Fault Protection with Tripping

Equipment ground fault protection is available as an option on MasterPact circuit breakers with MicroLogic 6.0A and higher trip units. MasterPact circuit breakers offer three different equipment ground fault sensing options: residual, ground-source return, and modified differential. The sensing options make it possible to match the number and location of current sensors to the application. The pickup and delay settings for equipment ground fault are adjustable locally with the dial settings or through the key pad. The pickup and delay settings for equipment ground fault are also adjustable remotely over a computer network on MicroLogic 6.0 and higher trip units. A neutral current sensor (NCT) must be installed in the neutral if equipment ground fault alarm is used on a three-phase, four-wire system.

Equipment Ground Fault Protection without Tripping

All MasterPact circuit breakers with MicroLogic 5.0P and higher trip units come standard with the ability to sense and report a equipment ground fault alarm through the optional programmable contact module or communication network. A neutral current sensor must be installed in the neutral if equipment ground fault alarm is used on a three-phase, four-wire system.

The pickup and delay settings for the equipment ground fault alarm are adjustable locally through the key pad on the trip unit or remotely over a computer network.

Residual Ground Fault Sensing

Residual ground fault sensing systems use one current sensor for each current-carrying conductor. The trip unit vectorially sums the secondary outputs from each sensor to determine if there is a ground fault and the magnitude of the ground fault. The following diagram shows the current sensors for a three-phase, four-wire system. There is a current sensor on each phase and the neutral.

Typical Residual Ground Fault Sensing System with Phase Conductors

The sensors for the phase conductors A, B and C are inside the circuit breaker. The neutral current transformer is installed in the neutral circuit. If the circuit breaker were used on a three-phase, three- wire system, the neutral current transformer would not be necessary.

Ground Source Return

Ground source return ground fault sensing systems use one current sensor on the ground conductor. The current sensor measures the ground current flow. The following diagram shows the current sensor for a three-phase, four-wire system. Ground source return can also be used on three-phase, three-wire systems.

Typical Ground Source Return Sensing System with Ground Fault Interface Module and Current Sensor

Minimum #14 AWG wire.

Wiring must be shielded cable or twisted pair.

Maximum of 500 ft (152 m) between ground fault interface module and current sensors.

Minimum #14 AWG wire.

Wiring must be shielded cable or twisted pair.

Maximum of 32 ft (10 m) between ground fault interface module and trip unit.

Ground-source return sensing systems require the use of the optional ground fault interface module and a sensor installed in the ground circuit.

The current sensor and ground fault interface module must be wired per the installation and wiring instructions included with the ground fault interface module.

Modified Differential Ground Fault System

A modified differential ground fault system (MDGF) is used for multiple sourced systems. Normal residual and ground-source return systems will not correctly sum all of the circulating currents caused by the multiple neutral paths and multiple grounds. The following diagram shows a typical main-tie- main system. Each source transformer is grounded, and the service entrance neutral is bonded to ground. Multiple neutral paths allow the neutral current to circulate and return to the supplying transformer by several different paths. The ground fault system must be capable of correctly summing these circulating currents.

Typical Modified Differential Ground Fault System with Ground Fault Interface Modules

Minimum #14 AWG wire.

Wiring must be shielded cable or twisted pair.

Maximum of 500 ft (152 m) between ground fault interface module and current sensors.

Minimum #14 AWG wire.

Wiring must be shielded cable or twisted pair.

Maximum of 32 ft (10 m) between ground fault interface module and trip unit.

The modified differential ground fault sensing system requires the use of ground fault interface modules and current sensors installed in all normal current-carrying conductors.

The current sensors and ground fault interface modules must be wired in parallel and the polarity of the current sensors must be maintained per the installation and wiring instructions included with the ground fault interface module.

High Resistance Grounded Systems

Where continuance of service for your distribution system is a high priority, high-resistance grounding adds the features of a grounded system on an ungrounded system while minimizing the risk of service interruptions resulting from ground faults.

High resistance grounding simultaneously provides a system reference point (or ground) to overcome the negative effects of low level ground faults and limits the magnitude of current that can flow during a ground fault. A resistance value is selected to limit the overvoltage during arcing faults. Industry practice has established that the resistance value should be selected relative to the system capacitive charging current.

An ammeter, in the circuit of a grounding resistor/grounding transformer assembly, reads the total amount of the ground current flowing through the system. When a current-sensing relay is added, any levels in excess of the capacitive charging current can be monitored. Alternatively, a voltmeter relay (not shown) can be connected across ground resistors to monitor the capacitive charging current in a resistor/grounding transformer assembly. The ground current ammeter provides a linear reading of the existent ac ground current present in the system for both the current and voltage detection methods described above.

Wye Systems Configured

High resistance grounding for Wye-connected systems is established by placing resistors in series with the neutral-to-ground connection of the power source. Grounding resistors are chosen to limit the ground current to a maximum value of five amperes. Line-to-neutral loads cannot be connected to a system where the neutral is resistance-grounded. For additional information, refer to 1999
NEC 250-36.

Four-Wire Wye System Fault Detection By Way of Current Relay

Delta Systems Configured

A neutral point must be established in an ungrounded Delta-connected system using three single-phase transformers. Typically, grounding resistors and transformers are chosen to limit the ground current to a maximum value of five amperes. This technique can be applied on Wye-connected sources when the neutral point is not accessible from the service entrance conduit. The neutral point cannot be used to service phase-to-neutral loads.

3 Wire Delta System with Zig Zag Grounding Transformers

Automatic Transfer Systems

Automatic transfer systems minimize power interruption by transferring the load from the normal source to an alternate source when the normal source is temporarily unavailable. The system uses multiple connections to power sources, usually utility sources, and a programmable logic controller (PLC) to achieve this transfer. These systems also feature redundant supplies of control power.

See examples of automatic transfer systems with Main-Main Circuit Breakers and Main-Tie-Main Circuit Breakers.

Main-Main Circuit Breakers

Each main circuit breaker connects to a utility source. When the normal source becomes unavailable, the system transfers to the alternate. If the system comes equipped with a preferred source selector option, the system reverts to the preferred source automatically once it is available. Without the selector, automatic retransfer does not occur.

Main-Main Circuit Breaker Configuration

Main-Tie-Main Circuit Breakers

Both main circuit breakers, connected to a utility source, are connected together by means of a normally open tie circuit breaker. Each main circuit breaker feeds independent load buses. Various settings of the preferred source selector switch and the retransfer on/off options determine which circuit breakers are closed during various operating conditions.

Main-Tie-Main Circuit Breaker Configuration

Special Applications

This section contains information regarding low voltage systems in seismic and high altitude applications.

Seismic Applications

QED-6 switchboards have been tested for applications according to Uniform Building Code (UBC) Zone 4 requirements. Equipment must be anchored properly to fully comply with Zone 4 installations.

High Altitude Applications

When QED-6 switchboards are installed at a location greater than 6600 feet above sea level, the ratings must be de-rated.

Altitude De-rating Factors

Altitude	Voltage*	Amps
6600 ft (2000 m) and below	100%	100%
8500 ft (2600 m)	95%	99%
13000 ft (3900 m)	80%	96%

NOTE: Ref. ANSI C37.20.1, QED-6 Switchboard Ratings.