Surge Protection for Building Controls

Surge protectors are relatively simple devices, yet must be properly selected and applied to function.  There are a couple of essential things to keep in mind when selecting surge protectors.

First, the operating voltage of the system is important.  Surge protectors are voltage-sensitive switches and must not clamp the system’s normal voltage.  The surge protector clamp voltage must be higher than the system voltage.  For example, a 24-volt DC system voltage usually uses a 30-volt surge protector.

Second, some surge protectors have an input side and an output side. If installed backwards, they will fail prematurely.

The following sections address some issues for surge protectors commonly used to protect Building Control systems.

The Protection Zone concept

Grounding is often misunderstood when it comes to the proper installation of surge protectors.  This can seriously affect the performance of protection systems and lead to damage electronics.  Here is a basic concept for surge protection systems that works!

The Protection Zone:

The protective zone is an imaginary circle drawn around and encompassing electronic equipment items that are located in close proximity to each other.  Everything passing through the imaginary circle should be commonly grounded and should have surge protection.

The Single Point Ground (SPG)

This is a common ground point, or node, used in the Protection Zone to bond all ground references inside the zone together.



The Protection Window

The protection zone window is a hypothetical “small opening” in the zone through which all electrical conductors enter or leave.  The window is located at the ground window.

Ground Conductors Must Be Short

Surge currents passing through a ground conductor generate a voltage across the conductor.  This is primarily due to the inductance of the wire.  Inductance is highly dependent on conductor length.  It is, therefore, very important to keep suppressor ground wires to the SPG very short.  The figure below illustrates a typical installation of equipment within a small area, such as an equipment room.  There are three problems associated with the installation depicted in the figure.

Problem #1: Multiple Grounds

There are three ground references in the above figure.  AC outlet #1, AC outlet #2, and the data line all present separate ground references.  The two AC outlet grounds are connected together at the power panel, many feet away.  The ground wire lengths offer enough inductance to effectively create separate grounds.  In addition, the data line may run hundreds of feet to yet another ground reference in remote circuitry.

Problem #2: Wiring Entering and Leaving Protection Zone

Observe in the figure the substantial distance between various conductors leaving the circle.  Even if ground conductors were bonded together, destructive voltages would exist during a surge due to wire inductance.

Problem #3: The Power Lines Lack Surge Suppressors

While the data line shows a surge suppressor, the lack of suppressors in the power receptacles leaves an open hole in the protection zone.  Even the best data line suppressor cannot prevent damage under these conditions.

Solving the Problems

The problems listed above are solved by employing the concepts described in the definition section above.  The figure below illustrates the proper modification of the installation.

  • Both devices are now powered from the same AC outlet.
  • The AC outlet incorporates a surge suppressor.
  • The protection zone ground is established at the AC outlet.
  • Data line suppressor(s) are added at the protection zone ground.
  • A ground bus bar is located at the ground area to facilitate multiple ground connections.
  • Ground wires to the suppressors are very short.

An optional, (depending on code), ground conductor connects the ground bus to the main building power ground.  This conductor may be quite long, but that does not create a problem, now that the ground area has been established at the protection zone.

Summary Rules

When applying surge protection, the following rules need to be observed:

1.  Keep all grounds inside the protection zone at the same potential.  If different ground potentials are present, damage will occur regardless of the suppression used.

2.  Protect all electrical and data circuits entering or leaving the protection zone at the protection zone ground.  This references all circuits to a single ground level.  This safe voltage is the  “clamp” voltage (let-through voltage) of the respective suppressors.

Practical Considerations:

The majority of sites are fairly simple and only involve bonding suppressor grounds to power grounds at the protection zone ground.  Existing sites may involve some rewiring to accomplish best results.  In order to keep the signal line surge suppressor ground and AC service ground very short, sometimes wiring must be moved.  For systems with signal lines, it is often easier to move the power service outlet to the vicinity of the signal line surge suppressors.

Protecting Multi-Drop DATA and Control Systems

Question: What is wrong with the diagram below?

Answer: The surge protectors (SP’s), located at the building entrance, are improperly positioned to protect the CPU and Electronics (E’s).  Remember that during lightning activity, ground potentials at opposite ends of a building (such as Building 2) can be thousands of volts.  This allows damage to E’s inside the building.

Also, surge protectors for data lines that enter buildings have series resistance.  The series resistance of the SP’s is additive.  Five protectors are encountered over the length of the data line in the above illustration.  The total series resistance often is too great and can cause communication or data line problems.

Question: What is the proper way to configure surge protection on a multi-drop system such as this?

Answer: Connect (locate) the SP’s on each individual E drop so that the SP is NOT in series with the main data line. The resistance’s of the SP’s do not add when connected as illustrated in the figure above.  In addition, it is preferable to locate the SP’s near the respective E’s to be protected.  This is especially important if the individual E’s are “plugged into” 120 VAC power.

Question: What do I do if there is no practical way to install an SP at each individual E’s such as a retrofit; or the E’s are powered down the cabling and there is no local ground at the E’s?

Answer: The above figure illustrates an alternate method of installation where the E’s in a given building are connected to the main data cable only at one point (at the SP).  If the individual E’s, are “plugged into” 120 VAC power, then it is essential that the SP for the building be located at the main power panel and grounded directly to its ground conductor.  The inter-building data cable should tee tap at the SP.  In all cases, it is preferred to locate the SP in the figure near the main power source.  Note that a SP always needs to be located at the CPU.

Installation of the PC642 Series

Proper Wiring of the PC642 is very important for it to work!

Question: Does it make a difference how the PC642 is wired into the circuit?

The left side is the strong side towards data lines from field. The right side is towards electronics to be protected.

Answer: Yes.  The PC642 must be wired “IN LINE” with your signal pairs.  This means that the pair must be “broken” and the PC642 placed in series with the pair.

Question: How do I connect the pairs to the PC642 base?

Answer: The protector has an unprotected side (strong side) and a protected side (fast side). The protector base consists of odd numbered (3,5,7,9) screw connections to terminate the field wiring (away from protected equipment). The protector base also consists of even numbered (2,4,6,8) screw connections to terminate the wiring to your sensitive equipment.

Question: After I wire the protector base, how do I know the PC642 is plugged in correctly?

Answer: A key pin (white nylon) should be located between positions 1 and 2 of the protector base (PCB1B).  Verify that the key pin is in the proper position.

Question: What happens if the PC642 is plugged in backwards?

Answer: You will experience premature protector failure, and possibly damage to your equipment.

Proper Grounding of the PC642

Question: I notice that screw connections 1 and 10 are both labeled “GROUND”.  Do I need to connect both to ground?

Answer: No, 1 and 10 are internally connected, so connect only 1 or 10 to ground.

Question: I have several PC642 protectors to install.  Can I daisy chain grounds, to screw connection 1 to screw connection 10 of the next unit?

 Answer: NO.  Each PC642 protector must have a ground wire connected to the local (at equipment) ground bus.

Question: Where do I connect the ground wire?

Answer: To the power ground (green wire ground) of the protected equipment.

Question: Does ground wire length make any difference?  If so, how long?

Answer: Absolutely, the shorter the better.  Keep the ground wire less than 1 foot.

Question: I understand that, but show me a convenient place to connect it?

Answer: A good question.  Implementing a short ground can be a problem, but it is critically important.  There are some power suppressors with a convenient ground lug built into the power suppressor (For example, the TS-1200G or the HSP-121-BT). The ground wire from the PC642 should connect to the ground lug on the power suppressor, or it should be bonded to chassis ground.  Make sure a good clean metal to metal contact is made, and make sure green wire ground is connected to the chassis.

Question: I still am not sure I can keep the ground wire less than 1 foot?

Answer: The data or signal pair may have to be rerouted to a location where the PC642 suppressor is located near the AC power outlet, or chassis ground.

Question: What about my panel that has no AC power supplied to it, power is supplied down the signal conductors. How do I ground the PC642’s?

Answer: Bond to the closest building approved ground point. Also, provide surge protection between signal conductors and the ground conductor.

Installation of the HSP-121-BT Series

Proper Wiring of the PC642 is very important for it to work!

The left side is the strong side towards input power conductors. The right side is the fast side towards electronics to be protected.

Question: Does it make a difference how the HSP-121 is wired into the circuit?

Answer: Yes.  The HSP must be wired “IN LINE” with your power conductors.  This means that the conductor must be “broken” and the HSP placed in series.

Question: What happens if the HSP is installed backwards?

Answer: The protector has an unprotected side (strong side) and a protected side (fast side). The fast side is not designed to handle large surge currents so it will fail prematurely and the HSP fuse will open.

Question: What voltage can the HSP-121 be used for?

Answer: The HSP series is designed for 120-volt power only.  Connecting it to 208 volts or 240 volts will destroy the HSP and open its fuse.  Blown fuses are not a warranty item, so insure the HSP is properly installed on the correct voltage, and that the circuit uses less than 15 amps.

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