Engineering Documentation

Chapter 1 – Wiring Regulations and Specifications
Grounding
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Single Earth Ground for AI, DI, and AO Circuits
● AI, DI, and AO circuits cannot be earth grounded at two points.
● The earth ground reference point on the controlling Building Automation System
(BAS) equipment is the only place where AI, DI, or AO can be earth grounded; this
is dependent on circuit design.
Equipment Grounding System Requirements
Earth Ground Reference
The earth ground reference for all field panels and equipment controllers must be
supplied via a third wire run, with the AC power source providing power to that
cabinet. All AC power sources must be bonded per NEC 250 unless isolation is
provided between the cabinets.
Equipment Grounding Conductor
The NEC and some building authorities allow the use of conduit as the equipment
grounding conductor. Field panels require a third wire or heavy wall conduit (with
threaded connections) for the equipment grounding conductor. In addition to an
equipment grounding conductor, you may use building steel or water pipes to bond AC
power sources if these are part of the earth grounding system approved by the Local
Building Authority.
When setting up an equipment grounding system, which consists of an equipment
ground connected to an earth ground, you must provide a third wire equipment
grounding conductor for any products of Siemens Industry. The equipment grounding
conductor must connect to neutral at only one point in the system; that point is the
neutral side of the transformer providing power to the equipment being installed. The
hot, neutral, and third wire conductors must all be contained in the same conduit (see
Figure Earth Grounding System [➙ 17]). This third wire may be connected to earth at
more than one point (that is, Siemens Industry does not require an isolated equipment
grounding conductor).
Grounding of Isolation Transformers and Standby Power Systems
The installation of isolation transformers and standby power systems follow the same
rules as equipment grounding requirements. Again, the neutral side of the locally
derived power system must be tied to the nearest approved earth grounding system.
NEC Article 250 Specifications
NEC article 250 states that the path-to-ground from circuits, equipment and metal
enclosures for conductors shall:
1. Be permanent and continuous
2. Have capacity to safely conduct any fault current likely to be imposed on it, and
3. Have sufficiently low impedance to limit the voltage-to-ground and to facilitate the
operation of circuit protection devices.
The NEC requires that all loads on a power source have their neutral side referenced
to the power source neutral and that the power source neutral be connected to the
earth grounding system at only one point. This is very important in preventing ground
loops. If building steel is not the shortest path, then you must use a water pipe or other
earth ground as designated by the local authority. You may still connect to building
steel, although the water pipe is your approved earth grounding reference; however,
you cannot connect from your source to steel, and then to the water pipe. Each wire
must be separate and of the correct gauge.