Headroom in a SPD
What It Is and What It Means To You

By Glenn Clifford

Senior Electrical Engineer

MCG Surge Protection, Deer Park, NY

(800) 851-1508

August 9, 2011

Ever since the IEEE Std. 587 was released back in 1980, “low clamp voltages” have been keywords in the surge protection industry.  Surge Protection Devices (SPD) are marketed to point to this very low clamp voltage in the hope that customers will assume that these must be the best solution to protect their valuable equipment and data.  Problem thus becomes, what affect does this close clamp voltage have on the life of an SPD and how practical is a low clamp voltage if it dramatically decreases protector life?

Ideally, a surge protector is purchased, installed and forgotten about. It should work quietly and effectively for 20+ years without a problem.  During its lifetime, the SPD should be clamping transients to a safe level, while causing no harm to equipment or data.  Designing a protector with a clamp voltage too close to the utility voltage is one way to prevent damage, but it’s short-sighted. In actuality, designing a protector in this manner makes for a good data sheet but ultimately the product is going to be particularly vulnerable to damage and a shortened lifespan. Sensitive equipment is now left hanging along the line, laid bare to any and all over voltages that course through AC and data lines everyday.

By typical industry standards, utility power is allowed to be plus or minus 10% of nominal for extended periods of time.  Utility companies (in developed nations) follow this and generally hold to it.  However, there are circumstances that are out of the control of the utility companies that can trigger unforeseen variations in power.  Sometimes for short periods, seconds to minutes, it is possible to see normal fluctuations in the order of 15 to 20%.  These can result from a variety of occurrences such as everyday utility switching, large inductive loads instantly either going on or off line like elevators and even traffic accidents involving power poles.

Surge protection companies design various protectors utilizing several different technologies.  By and far the most reliable protectors use MOV technology for AC power applications.  An MOV is a metal oxide varistor which is used in shunt to divert transients away from sensitive electronics.  Under normal conditions an MOV appears to the circuit as a high impedance connection until the voltage threshold is reached.  At that point the MOV becomes a short circuit “clamping” the over voltage down to a safe level.  When the voltage returns below the threshold level, the MOV reverts automatically to a high impedance connection.

For transients, high voltage anomalies lasting generally less than 500us, the MOV operates over and over again with very little degradation as long as the current level is below what the MOV can handle or there are enough MOV sharing the transient that the current remains below what each MOV can withstand.  MOVs offer clamping characteristics close to an ideal clamping device (Littelfuse AN9768 January 1998 Transient Suppression Devices and Principles p. 10-106) while still offering high-energy capability along with a relatively low price.  There are many different clamping devices available but none measure up to the ability of the MOV to clamp high energy transients commonly observed on power lines.

The problem where many surge protectors become damaged is either when a protector is undersized or more often, when the MOV finds itself clamping longer duration surges.  This is where headroom now becomes critical.

Headroom can be explained as the difference in voltage between the peak of the sine wave and a higher voltage level where the MOV starts to turn on. If the difference between the two is too small, the MOV may conduct more frequently, resulting in a shorter MOV life. Headroom margins of 15% and greater address this issue with virtually no effect on suppressor performance. In some areas, power line fluctuations can exceed 15%. A good SPD design requires higher clamp voltage MOV in these situations.

It is not only 120V systems that are at risk; systems such as 277/480Y are also at risk of fluctuation.  Recently MCG encountered a problem with a unit installed on a 277/480V line.  The building was under construction and the power was being taken on and off line periodically.  This caused voltage swings most likely in the order of several cycles.  Standard SPDs were being damaged.  An MCG protector utilizing high headroom MOVs rode through it all, continuing to protect equipment from the real hazards of high amplitude transients.

A standard MOV has a tolerance of +/-10%.  That translates to a MOV with a published MCOV of 320V could come in as high as 352V at +10% or as low as 288V at -10%.  Assuming we have a MOV with -5%, which is not unreasonable,  that gives an MCOV of 304V a headroom of only 27V.

Here are some possible power fluctuations:

            277V + 10% = 304.7V

            277V + 15% = 318.5V

            277V + 20% = 332.4V

With this data it is easy to see that a 320V MOV with -5% or 304V will be too close for comfort on the 277V + 10% but will probably survive without any incident, even for long periods of time. The outlook for the 277V + 15% is not so favorable.  Although this may be less common in countries or areas with well-regulated power, there are circumstances where it will occur. This MOV will fail if the voltage remains constant longer than just a few minutes.  It will definitely begin to degrade if it is just clamping each peak of the sine wave for a short time.  Finally, the 277V + 20% is going to cause the MOV to overheat and degrade significantly or just fail outright even in as short an amount of time as 1/2s.  Although this is not a very likely scenario, it can and does happen from time to time with devastating effects to any surge protective device that is trying to impress with low clamping results on its data sheet.

Higher headroom does not present a problem to equipment.  The Information Technology Industry Council (ITI) publishes a curve that shows the susceptibility of typical business equipment.  In the industry it is known as the ITI (CBEMA) Curve. This curve was made for single phase 120V equipment, the most vulnerable to transients, but can be extrapolated for any voltage.  Refer to Figure 1.  This curve clearly shows that equipment can withstand higher transient voltages for very short durations.

A higher headroom approach, for example, uses a 390V MOV on 277V systems.  In a “worst case” intendment, an MOV at -10% is applied. That equals 351V, still well above the 332.4V that can be seen with utility +20%.

In conclusion, when protectors are designed to clamp too close to the utility power, the outlook for downstream equipment is poor.  The protector becomes damaged prematurely and oftentimes, leaves the facility and equipment unprotected until the protector is repaired or replaced.  Using higher headroom MOV does give a slightly higher clamp voltage but it is still well within guidelines to protect equipment for short durations of transients, typically much less than 500us. 

Along with this common-sense design innovation, adding a low inductance cable can lower the voltage drop up to 1/3 of standard wiring.  Using multiple Neutral wires alongside the phase wires and twisting the wires tightly along the run is one way to accomplish this.  This two-prong approach offers excellent surge protection performance. Adding a 20-year product warranty and lifetime on protection modules further ensures maximum equipment uptime.


Glenn Clifford is a Senior Electrical Engineer at MCG Surge Protection. The company has exclusively been in the surge protection business for over 40 years.