MONITORING OF SURGE-PROTECTIVE DEVICES 

IN LOW-VOLTAGE POWER DISTRIBUTION SYSTEMS 

by 

James Funke 

Chief Engineer 

Eaton/Cutler-Hammer 

François Martzloff 

Principal 

Surges Happen! 

1. Introduction 

Surge-protective devices (SPDs) are installed at most industrial or commercial facilities to protect 

equipment against transient events. These SPDs can be located anywhere in the electrical distribution 

system. Because these devices are primarily connected in parallel (the so-called “one port” type), 

failure of the SPD does not necessarily cause any immediately noticeable symptoms for the user. 

Many of these devices are not inspected once they are installed and the user is not aware of their 

operating status. Therefore monitoring becomes essential in maintaining the surge protection within 

the facilities. There is a lack of thorough information on monitoring of SPDs, especially if looking 

beyond the visual indicators. 

 The monitor is a critical part of the SPD as it tells the world if the SPD is working. If the surge 

protection were removed from a location, there would be no immediate change to the electrical system 

that would affect the occupant. However, if a large surge event then occurred, there could be many 

damaged systems in the building. The monitor can show that the SPD is connected and that it is 

operating correctly. Nevertheless, the SPD still requires some type of maintenance system to 

systematically inspect each SPD unit and determine its operating status. 

 

The monitor can be visual, for instance green and red Light Emitting Diodes (LEDs), audible, or 

electrically connected signals. Visual signals are the most common type encountered in the field. 

There can be just one light or a whole array of lights indicating the functionality of various parts of th

product. Audible alarms are useful in a low-traffic area. Both of these types of annunciators are only 

useful within a small local area. Local or remote electrical monitoring has been done primarily by dry 

contacts that change state in the event the SPD fails. This signal can be routed to a building system or 

a remote location. 

In this paper, we focus on the concerns of permanently-wired SPDs in industrial or commercial 

facilities rather than the application of plug-in consumer-type SPDs. 

2. Types of Monitors 

Manufacturers offer a vide choice of SPD monitoring approaches, raising questions of pros and 

cons depending on the application needs. The following paragraphs provide a brief description of 

some possible approaches, summarized in Table I. 

A. No Monitor 

Some SPDs, such as most surge arresters, do not have any type of explicit monitoring. Although 

some arresters have a thin white paper sheet that burns or is damaged during a fault (which is really
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

a form of visual indication), most models rely on the user to notice a ruptured case or tripped circuit 

breaker. The issue is that without a detailed visual inspection, the facility can be unprotected for a 

significant length of time before someone notices. This is also true for surge protection built into mos

power supplies used to protect the electronics they supply. 

TABLE I 

SUMMARY OF LOCAL MONITORING FEATURES 

Type of 

monitoring Features Advantages Limitations 

No Monitor None No electronics to fail Cannot determine status of un

Visual 

Indicator 

LEDs / neon lamps Immediately visible to anyone 

looking at the unit 

Non-standard meaning of lights

on/off can be misinterpreted as 

no monitor 

Audible 

Alarm 

Buzzer / horn Immediately noticeable to 

anyone near the unit 

Can be annoying, might be 

silenced and then ignored 

Electrical 

Indicator 

Form C relay / 

optoisolators 

Remote monitoring, integration 

into building automation systems 

Limited information 

Surge 

Counter 

Surge counting Actual count of surge events No reference level to compare 

Other 

Counters 

Sag / swell / outage 

and others 

Indication of system power 

quality 

No reference level to compare 

Protection 

Remaining 

Internal component 

counting 

Indication of life remaining on the 

unit 

Many failures are due to 

exogenous events and not 

predicted by life remaining 

Event 

Recording 

Date/time stamps, 

event values 

Correlation of monitoring and 

external events, level of event 

Accuracy not as good as a 

power quality meter – but that 

not the purpose of SPD 

monitoring 

B. Visual Indicators 

Other SPDs have included monitor lights that indicate their operational status. At the most basic 

level, these lights are used to tell the user if the SPD has power or not. There can be one light or one 

light per phase wire. The light used is usually either a light-emitting diode (LED) or a neon lamp 

although other types are possible. 

Many failures of SPDs, especially those not provided with an internal disconnector, result in a shor

circuit that trips the upstream circuit breaker, making it obvious that something is wrong, but resetting 

the breaker might not necessarily pinpoint the problem to the failed SPD: In the case of an SPD with 

internal disconnector, the coordination between the disconnector and the upstream breaker might be 

such that both will disconnect. Then, resetting the breaker will return the power system to “normal” 

and the user might think that all is well – but no surge protection is provided, hence the need for a 

clear indication. 

Depending on the design of the specific SPD, the user might get an on/off indication. However, 

there is a problem in that the maintenance staff might not fully understand the differences in the 

meaning of these lights and thus not notice the failure of the unit (Martzloff, 1998 [1]). Further, neon 

lights can burn out after a year or two and transform the unit to a model with no monitor as describe 

above. 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

More advanced models include a two-state light, usually LEDs that have a longer life than a neon 

lamp. These models use a green/red indication. Normally the green LED will be illuminated telling th

maintenance staff the unit is operating normally. If the unit faults, the unit will turn off the green LED 

and turn on the red LED. Some of the models can have more than two states. By adding a yellow LED

a three-state version can be created. The actual meaning of the yellow LED varies by manufacturer. 

Like the on/off indicator above, this version can have a multiple state indicator per phase or just on

for the complete unit. Because the meaning of these lights is not standardized, there is the possibility 

of misunderstanding their meaning unless clear explanations are conspicuously provided next to the 

light. More complex monitors would also require text within the instruction manual to fully describe 

the functioning of the monitor and the meaning of all possible combinations of lights beyond that 

indicated on the product itself. 

C. Audible Alarms 

The problem that can occur with any type of visual indicator is that people might not notice or pay 

attention to the signal. Audible alarms are much more difficult to ignore. However, a noisy 

environment can easily swamp the small buzzers that are common in SPD monitors. In most 

conditions, especially normal office environments, the sound can be heard through in the hallway nex

to the electrical room, even through a wall or closed door. Options included for the audible alarm can 

be important for some users, for instance: 

3⁄4 Can the buzzer be silenced after the unit has faulted? Imagine sitting next to a faulted SPD 

for several days until it can be replaced. 

3⁄4 If it can be turned off, is it obvious to nearby personnel whether the alarm is active or in 

silence mode? 

3⁄4 A power outage will result in the SPD losing power and then starting up again after the 

power returns. Does the SPD return to the previous condition, alarm silenced or not, or does 

the SPD start up in the same state every time? 

3⁄4 Assume that the unit has the alarm silenced as default, the maintenance personnel will need 

to reset the alarm silence function after every power outage. 

D. Electrical Indicators 

The most common type of electrical indicator is the Form C relay. This relay has one set of contact

that include a normally-open, normally-closed contact with a common terminal. There are various 

different actions for this type of monitoring. 

The first question with a Form C relay is what is considered the normal position? Although the 

normal condition for an SPD is powered, relay markings are the opposite, ‘normal’ is unpowered 

condition. The person using the relay in a building automation system must connect the unit in such a 

way that it tells the system that it is not faulted when powered and operating normally. 

The second question to ask is if power is lost, does that relay switch or not? It can be assumed that 

the relay will reverse conditions under a fault condition, that is, the normally open terminals will 

become closed while the normally closed terminals will become open. However, it is not clear what 

happens during a power failure. Is the power loss considered a fault where the relay changes state, or 

is the power loss normal, and the monitoring system therefore sees no change? Answers to these 

questions should provide the building maintenance group with enough information to understand the 

operation of the relay. 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

Although connecting to building automation system is one of the more common uses of the Form C 

relay, it is not the only use. With the help of an external power supply, a large red lamp can be 

connected through the normally open circuit. If the unit changes state, the relay will close and turn on 

the lamp that can then alert the maintenance staff of the problem. In some environments, a horn can b

used to announce the fault audibly. 

Besides the Form C relay, there are other ways to annunciate the status. An optoisolator with a 

transistor output is one example. This example requires an external circuit to interface with the 

transistor to couple to a building automation system or power an external lamp. Some units offer only 

a normally open or normally closed contact, while other manufacturers offer multiple contacts. 

It is essential that the electrical ratings published by the manufacturer be observed. Relays have 

maximum voltage, current, power and VA ratings that should not be exceeded. For dry contact 

monitoring, this is not an issue due to the low power and voltage associated with dry contact 

monitoring, but when connecting external loads, the ratings can easily be exceeded and the load can 

damage the relay and render the SPD and its monitor inoperable. In the worst case, the relay can weld 

in one position and be unable to indicate the proper function. 

3. Other Monitor Features 

A. Surge Counters 

A surge counter is a very common feature that was added to high-end monitors over the last decade

Users have often questioned the need for providing an SPD in a power distribution system. A perfect 

SPD will have no effect on the distribution system except during a transient event. During the surge, 

the SPD should reduce the impact on the distribution system such that there is no noticeable change. 

So a perfect SPD would be unnoticed by an end user and therefore raise the question of its need. At 

the same time, a site with no surge protection and few transients will behave in the same manner. 

The solution to this lack of proof was a surge counter that records the number of times a transient 

event occurs. This is an effective method of showing that the SPD has actually been doing something. 

However, as the need for surge protection became more widely recognized, providing the proof of 

their operation became less important. 

A surge counter can measure a variety of different physical phenomena. The monitor can measure 

the voltage and record voltages over a preset level as a surge. It can also measure the current and agai

anything over a preset current level is considered a surge. Surges are a high-frequency event. A 

frequency-dependant device can also be used to record a surge. In general, surge counters use a 

combination of frequency and voltage or current. An event that is high in frequency and voltage or 

current is very likely to be a surge and will be counted. Other ways of detecting a surge can be used, 

such as Hall effect sensors recording the magnetic field. 

There are different sensitivities (thresholds) of these surge counters and this makes it difficult to 

compare the actual number of counts to those recorded at other sites (Martzloff and Gruzs, 1988 [2]). 

More important would be a change in the number of counts per period. For example, a site that has 

had 3 surges per month over the last year and then changes to 23 counts in the last month should be 

looked at to determine what has changed in the past month. 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

B. Other Counters 

A more recent addition to SPD monitoring has been the inclusion of counters for other power 

quality events. The most common counters have been capable of detecting sags, swells and outages 

(IEC 61000-4-30, [3]). Over- and under-voltages and dropouts have also been included in some 

monitors. They differ from the first three by the duration of the event. 

Similar to the simple surge counter, for a power quality monitor, the most important attribute is not 

the number of events, but a change in the number of events. Assuming that a facility is operating with 

few problems, a small number of events can be considered as the nominal background noise and 

changes to this level are significant. Some locations might be experiencing trouble before the 

installation of these SPDs. In this case, the highest counts recorded can be used to begin diagnosing a 

distribution system problem. For example, at a site with 5 sags, 3 outages and 117 swells, one should 

be looking at problems dealing with abnormal high voltage and using equipment that can measure 

swells more accurately. Perhaps the incoming transformer is tapped too high and as loads are shut off

frequent swells occur. 

An SPD with counters capable of recording different types of events should not be mistaken for a 

power quality analyzer. It can help indicate a power quality problem but it will not solve the problem 

alone. It is better to view it as a power quality indicator. However, the monitor can show, for instance

that the majority of counts are sags. Further, if the equipment having a problem is susceptible to 

voltage sags, this can indicate that a sag-mitigating device will improve the situation. The particular 

benefit of an SPD with multiple counters is that the SPD is needed for surge protection anyway and 

that it is always connected to the distribution system, constantly recording events. If a problem 

occurs, it can be reviewed to look for trends in the power to help choose what equipment to use to 

better measure the system or rule out a possible cause. Its limitation is that the device must be 

monitored on a systematic basis to get the most use out of its features. 

C. Protection or Life Remaining 

Ideally, it would be desirable to have an SPD that never fails. Unfortunately this would result in a 

product that is too expensive and would be too high in voltage protection levels to adequately protect 

downstream equipment. Alternatively, it would be very desirable to predict exactly when a unit would 

fail or need replacing. Again, this is not possible. 

For small SPDs, the indicator lights are a form of remaining life indication. If the light is green or 

on, the life is 100%. When the light changes to red or off, the life remaining is 0%. Larger units utilize 

multiple internal surge components. Each component can be fused or monitored by some method. 

Then an effective life remaining function can be implemented by counting the number of active 

internal components and dividing by the nominal number of internal components. This scheme can be 

used to provide information for a green/yellow/red monitor or to provide a percent-remaining feature. 

Another way to implement this feature is to measure one aspect of the surge protection such as a 

voltage let-through, leakage current, or voltage at a fixed current. This is more difficult to implement 

in practice as the effects of the distribution system must be removed and this will require the 

disconnection of the SPD from the system in order to measure the value. The additional maintenance 

and time means that it will rarely be performed consistently and repeatedly. 

There is also a problem with the failure boundaries. Due to the non-linearity of the components in 

SPDs, there can be little change in these attributes until the event that causes them to fail. This is also 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

a problem with the component counting method as well as the aspect measuring method. Events that 

cause a failure can damage a brand new product as easily as a product installed for years. The SPD 

will show 100%, right up to the event that destroys the unit. 

D. Event Recording 

An improvement to the counter on some models is the recording the magnitude of an event and 

time/date stamping of the event. This produces two clear and immediate benefits. First is the ability to 

go back in time and look at exactly when each event occurred. With a simple counter, the user would 

be required to record the amount of sags that occurred in each time period other-wise there is no way 

to tell if an event occurred last week or last year. If a machine was shutdown yesterday, with an event 

recording, the user can review the log to see what events had occurred just prior to the shutdown. 

Finding that a swell occurred within one second of the shutdown is much more useful than noting that 

one swell had occurred sometime in the last month. 

The second benefit is the recording of magnitude. An 80 Vac sag on a 120 Vac system would be 

much more significant than a 108 Vac sag. Recording the event that shutdown a machine would be 

extremely helpful in fixing the problem. Event recording is much more useful than event counting, bu

it is still not a substitute for knowledgeable staff that can work to track down power quality issues. 

4. Remote Monitoring 

SPDs can be installed in electrical rooms and distant locations where there is not necessarily 

maintenance staff. This creates a need for monitoring from a remote location. There are several 

methods in use for observing an SPD at a distance. Manufacturers offer a choice of remote monitoring 

approaches, raising the questions of pros and cons depending on the application needs. The following 

paragraphs provide a brief description of possible approaches, summarized in Table II. 

TABLE II 

SUMMARY OF REMOTE MONITORING FEATURES 

Monitoring type Features Advantages Limitations 

Direct 

Connection 

RS232, RS485 Full information from the unit Limited distance, custom 

software, 

Addressable 

Relay 

Form C relay to 

addressable relay 

Connections to building 

automation software, no 

distance limitation 

Limited information from 

unit 

Network 

Connectivity 

LAN, WAN or control 

network connection 

Full information over any 

distance and flexible network 

connection 

Requires computer 

access – increasingly a 

standard feature in new 

facilities 

A. Direct Connection 

By far the most common type of connection, this is usually achieved by connection to the Form C 

relay. The amount of information is limited and is often only the status of the unit, operating or not. 

Some models have been able to transmit the information on each phase and send three separate signal

to a remote station in the same building. The manufacturer that supplied the SPD can also supply this 

circuit. Then the optoisolator connection discussed above can be utilized by connecting to a dedicated 

product intended to supply remote information. However, the amount of information that can be sent 

is limited and the distance is generally limited to the same building as the SPD. 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

B. Addressable Relay 

A sophisticated building automation system might not be able to directly connect to a Form C relay

To connect to building automation system, an addressable relay can be used. This is a device that can 

be addressed on a network and can read the status of a Form C relay. This allows the Form C relay on 

the SPD to be seen anywhere on the network. With the presence of wide area networks, this allows th

status of the SPD to be determined anywhere in the world. However, the amount of information 

transmitted is very limited in this application. 

C. Network Connectivity 

For the most complete monitoring of the information that the SPD can provide, a network 

connection is required. Such connections generally fall into three categories: local connections such a

RS232/RS485, industrial control systems such as Modbus, and local area networks such as Ethernet. 

For the most part, the differences in these networks are not important. The differences are in the 

implementations of the connected hardware and software. The presence of a network connection 

allows information to be transmitted anywhere it is needed. Modbus would likely be preferred in 

industrial settings while Ethernet connections would be more common elsewhere. 

RS232 and RS485 will require a product-specific computer program to properly transfer the 

information in the SPD to the computer system that is eventually involved. Modbus and similar 

protocols require a driver specific to the SPD and the protocol. Finally, an Ethernet enabled SPD can 

be designed to use TCP/IP and connect directly to a computer by using a Web browser such as 

Netscape Navigator or Internet Explorer. The information is written into the SPD as a Web page that 

can be downloaded as needed without need for additional software on the host computer. 

The above approaches provide a method of getting the information the SPD has collected into the 

user’s ‘hands’. However, only the last approach, using an imbedded Web page, allows for seamless 

computer upgrades and software upgrades. Since the viewing software is now effectively part of the 

operating system, the new computer will always have the required software without the need to 

separately install the SPD software. Computers are changed every two to three years in a business 

environment and often upgraded even more frequently. This situation creates a problem, as the SPD 

software might not work on the new computer or with the upgraded software. The other benefit of the 

Ethernet connected version is the ability to update the firmware by downloading a new image into the 

SPD if there are ever any issues that need to be resolved in the future. 

5. Disconnectors and Monitoring 

The importance of providing monitoring that satisfies the mission of the SPD is influenced by the 

decision of the power system designer (specifier) in view of the present ambiguity of the requirement 

for internal vs. external disconnector function. 

For instance, the IEC definition of SPD disconnector (IEC 61643-12:2002 [4]) mentions a “visible 

indication of SPD failure” but a note (not mandatory) states, in part, “Each disconnector may be 

integrated into the SPD or external to it.” (Note the use of “may” as meaning permission according to 

IEC style.). Further on in the referenced IEC standard, the following sentence appears: “ ... It is also 

necessary to be sure that the adequate disconnector or back-up protection as suggested by the 

manufacturer is present and working...” Accordingly, the choice of internal vs. external is left to the 

discretion of the specifier. 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

Unlike the consumer-type SPDs market of two-port SPDs where one can find either a paralleldisconnector that merely disconnects failed SPD elements but leaves the load powered and 

unprotected or a series-disconnector that maintains protection by disconnecting the load (Martzloff, 

1998 [1]), most hard-wired SPDs in power distribution systems are one-port (parallel, shunt) devices 

so that an internal disconnector will indeed remove a failed SPD from the power distribution system, 

leaving the loads powered but unprotected. 

There are some SPD designs intended for surge and temporary overvoltage protection of unattended critical assets 

that intentionally do not contain a disconnector but instead positively make the failed SPD a very low impedance to 

cause tripping of the upstream breaker and thus activate an on-site standby emergency generator, avoiding the risk of 

leaving critical loads exposed to incoming surges with a disconnected one-port SPD. Remote monitoring the SPD 

status can alert system operators before the fuel storage of the emergency generator is exhausted.

6. Failure of Monitors 

Monitoring of some sort is required in most applications to determine the status of the unit and to 

keep a facility fully protected. Yet it comes with a price to pay, both literally and figuratively. A 

tradeoff must be made between the needs of the location and the cost of the additional features. 

Further, adding complexity in the form of a monitor will reduce the reliability of the SPD as a whole. 

Given most applications, this is an acceptable or even required compromise. A good warrantee 

becomes more important with more complex monitors as the reliability, based purely on parts count, 

can decrease with higher-end monitoring. This is also true of products of all types and not just limited 

to SPD monitoring. 

This brings up the topic of how does a monitor react to monitor failures. Some products will 

default to show a ‘normal’ signal while others will indicate a fault. The logic within the monitor can 

be done either by normal indication that is pulled to show a fault or a fault indication that is pushed to 

show a normal condition. In the first case, a fault in the circuit that is responsible for pulling the 

signal to show a fault will result in a normal signal even though the monitor has failed. In the second 

case, the same fault will result in a fault signal, as this is the default condition for the monitor. The 

difference between these two is only apparent when the monitor itself has failed. The pulled signal 

will show that everything is normal while the pushed signal will show a fault on the monitor. This 

function is very difficult to assess in field or to demand in a specification. Complete discussions of 

this aspect of monitoring are well beyond the scope of this paper, other than to point out this potential 

issue. This issue will not affect most users, but it can become important in operation-critical 

applications where a false positive signal could leave a sensitive location unprotected against surges 

for extended periods of time. 

7. Monitor or SPD 

The monitor is the SPD itself for most end users. Purchasing of SPDs is often based solely on the 

monitor rather than on the surge suppression function. Many specifications deal primarily with the 

monitor and only casually mention the presence of surge suppression. The appearance and features of 

the SPD monitor can overshadow the primary purpose of the SPD, which is to protect equipment 

against damage and operational errors caused by surge events. The SPD should be chosen first for its 

ability to protect the equipment and only then for its monitoring capability and features. SPD 

monitoring options should be chosen based on the needs of the facility, the needs of the users, and the 

ability to use the data. Future needs should also be reviewed as adding features later can be difficult 

or impossible without replacing the complete unit. 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

8. Examples of monitors 

As mentioned above, there is a wide range of monitor functions and hardware offered by 

manufacturers. Figure 1 is an example of a local visual display with functions limited to a “keep it 

simple” philosophy. Figure 2 shows for a remote Form C set of contacts, and Figure 3 shows the 

complexity of hardware (and implied firmware and software) associated with a remote-reading 

function. 

Figure 1 – Typical two color LED display (all green here) and audible 

alarm disable with visual indication of audible alarm being disabled 

Figure 2 – SPD with Form C relay contacts terminals shown and output 
MONITORING SURGE-PROTECTIVE DEVICES 

by James Funke and François Martzloff 

Figure 3 – Example of the complexity of a high end monitor with internal microprocessor, 

memory, battery backup, Ethernet, Modbus, LCD display, buzzer, Form C 

9. Conclusions 

Industry is offering a wide choice of monitoring methods and hardware implementations to provide 

the users and operators of distribution power systems with timely information on the status of installed 

surge-protective devices. 

It is therefore important that at the design stage of a facility, or as later retrofit installation, the 

designers (specifiers) of the SPD package and monitoring hardware work in concert with the ultimate 

user of the facility to provide an approach that will satisfy the needs of the mission. 

The authors hope that this paper and its summary tables will assist power distribution system 

designers in this important selection process. 

10. References 

[1] Martzloff, F.D., “What Are the Lights on Your Surge Protector Telling You?” Power Quality 

Assurance, July/August 1998. 

[2] Martzloff, F.D. and Gruzs, T.M., “Power Quality Site Surveys: Facts, Fiction, and Fallacies,” 

IEEE Transactions IAS-24, November/ December 1988. 

[3] IEC 61000-4-30 (2003-02): Electromagnetic compatibility (EMC) - Part 4-30: Testing and 

measurement techniques - Power quality measurement methods. 

[4] IEC 61643-12 (2002) Surge protective devices connected to low-voltage power distribution 

systems – Selection and application principles. 

-o-o-o-o-o-o-o-o-o-o-