Monthly Engineering Horizons



May 22
10:37 2019

Monitoring the Integrity of Engine Start Signal Circuits

Strategies for Complying with the New NEC® Requirement

The 2017 edition of the National Electrical Code® (NEC®) introduced a requirement to continually monitor the integrity of generator start signal circuits. This paper explains how unmonitored wiring presents a reliability risk, and describes the responding NEC requirement. It also describes ASCO’s solution for complying with the code, without having to rewire existing power equipment control circuits.



In order to provide an engine start signal to an emergency generator, a transfer switch typically uses a circuit controlled by a normally closed contact. During normal operation, this component is energized and the contact remains open, but closes when the Automatic Transfer Switch (ATS) requires generator power.Upon closure, the contact completes the circuit used to send a start signal to the engine of a standby generator. The engine start signal wiring is shown in Figure 1

Figure 1: When utility power fails, the contact de-energizes and closes and  a start signal is sent to the engine.

A problem occurs when the generator control wiring becomes discontinuous, resulting in an open fault as shown in Figure 2. This could result from modifications to the normal or emergency power systems, or when damage occurs such as accidental cutting of a conduit or wire. In these instances, the engine controller would not detect the fault because it still sees an open circuit. The problem would not become apparent until the engine failed to start following a utility power outage.

Alternatively, if a short were to develop in a system that uses a normally open contact (which is held closed when generator power is unnecessary), the engine would not start even if normal utility power is unavailable. Because of susceptibility to this type of fault, the use of a start circuit equipped with a normally open contact would not fully comply with the NEC requirement. Figure 3 shows a short along a circuit equipped with a normally open contact.

Figure 3: If a open-to-start signal is used, a short between the conductors could remain undetected until the generator fails to start.

The National Fire Protection Agency recognized the potential for damaged engine control wiring to impact generator availability, and addressed the issue in the 2017 NEC. Article 700.10(D)(3) reads as follows, with the added requirement shown in bold text:

Control conductors installed between the transfer equipment and the emergency generator shall be kept entirely independent of all other wiring and shall meet the conditions of 700.10(D)(1). The integrity of the generator control wiring shall be continuously monitored. Loss of integrity of the remote start circuit(s) shall initiate visual and audible annunciation of generator malfunction at the generator local and remote annunciator(s) and start the generator(s).

Compliance with the new requirement thus requires three elements:

  1. Continuous monitoring of generator signaling circuits
  2. Visual and audible annunciation of changes in state
  3. Automatic transmission of an engine start signal and alarm when a problem is detected


To develop a solution that is easy to implement and complies with the NEC requirement, several approaches could be used to monitor the integrity of engine start circuits. However, many solutions have at least one characteristic that does not fully conform the new NEC requirement. For instance, a three-wire system with both normally open and normally closed (Form C) contacts could be used, as shown in Figure 4. The states of indication for this arrangement are shown in the corresponding table.

Figure 4: Use of Normally Closed and Normally

Open Contacts

Using this solution, the two contacts should always be in opposing states. A power failure would close the normally closed contact and open the normally open contact to issue a start signal, resulting in an engine start. However, if both contacts are in the same state, then a problem exists in the start signal circuit. If an open were to develop along the start signal circuit, as shown in Figure 5, an abnormal condition may not be detected because the contacts would appear to be in a valid state. This condition would remain until a power outage resulted in an actual engine start. In practice, this system would not continuously monitor the integrity of the circuit because the short would not be annunciated when it develops. For this reason, the arrangement would not comply with the NEC requirement.

Figure 5: An open along the start signal circuit could go undetected by the normally closed contact.

For more information: ACO Power Technologies






About Author

Engineering Horizons

Engineering Horizons

“Engineering Horizons” is the first & leading technical magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics field under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.

Related Articles

Latest Magazine


Please subscribe FREE e-magazine

As you know, monthly “Engineering Horizons” is the first & Leading Technical Magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics fields under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.
WhatsApp WhatsApp Live Chat