Start with the conclusion: running does not mean protection is intact
After a thunderstorm passes, the most common on-site judgment is "the equipment is still running, the lights are still on, data is still coming back, so it should be fine." That judgment answers only whether the equipment's **function** survives; it does not answer whether the equipment's **protective capability** survives. Surge protective devices and similar protective components do not take part in day-to-day operation. Their role is to divert energy when overvoltage or lightning current arrives. Once the protective action completes, the protected equipment usually shows no visible abnormality, while the protective device itself may already have degraded. In other words, equipment that still runs does not mean the protection stage is undamaged.
Among the SPD monitoring elements, leakage current, temperature, and lifetime estimation are explicitly listed as status quantities. These quantities matter because they correspond to "how much protective capability is left," not "can the equipment still be used right now." Whether these status values need to be read after a thunderstorm is essentially asking: when the next surge arrives, will the protection still be there?
1. Why post-thunderstorm degradation is often hidden
The first reason hidden degradation occurs is that "function is normal" and "protective capability is normal" are two different evaluation standards. Equipment function is determined by power supply, control, and communication stages, which are usually designed with redundancy; the degradation of an individual protective component will not immediately interrupt production or trigger an alarm. After a thunderstorm, therefore, operators see equipment running as usual, but do not see that a link in the protection chain has weakened.
The second reason is that degradation of the protective component itself is cumulative. From engineering experience, lightning strikes or long-term operation cause SPDs and similar protective components to degrade gradually. Degradation is not a single-point event that happens suddenly after one strike; it is a process that advances slowly with operating time and the number of surges. Precisely because it is slow, it is more easily masked by the appearance that "the equipment is still normal."
The third reason is that peripheral links such as grounding can also fail. Taking grounding resistance abnormally open-circuit as an example, it is listed as one of the trigger conditions, on the basis of GB 50057. Once the grounding link is abnormal, the discharge path of the protective device is no longer complete, and this change likewise does not appear directly in the equipment's operating state. Post-thunderstorm checks therefore cannot focus only on whether the equipment still turns; they must also look at the protection and grounding links that "make no sound in normal times."
2. Three classes of status quantities for judging degradation
Since "whether the equipment runs" is insufficient to judge protective capability, quantifiable status quantities are needed. Among the SPD monitoring elements, leakage current, temperature, and lifetime estimation are three representative status quantities.
The first is leakage current. Taking the FS surge protective device monitor (FS-00011-R) as an example, its key parameters give a leakage current range of 50.0–1200.0 μA and an accuracy of ±10 μA. Leakage current reflects the small leakage current of the protective device under normal voltage; it is a measurable with a range and an accuracy, not a qualitative judgment by feel.
The second is temperature. Temperature is included among the SPD monitoring elements. Compared with leakage current, temperature is more of a quantity that must be interpreted together with the installation location and actual operating conditions; its value lies in indicating whether the protective device or its connection points have developed an abnormal thermal state.
The third is lifetime estimation. Its key parameters also give a lifetime-estimation range of 0–100%. The significance of this status quantity is that it turns "how much longer can it be used" from an empirical judgment into a readable percentage, providing a basis for replacement decisions.
Taken together, the logic is clear: leakage current answers "how much is leaking now," temperature answers "how hot is it now," and lifetime estimation answers "how much margin remains overall." All three serve one purpose—seeing the attenuation of protective capability in advance, while the equipment is still running.
3. How monitoring elements map to specific products
The value of status quantities ultimately has to land on products that can be installed and read. Under the theme of lightning risk and safe operation of lightning-protection systems, the typical application scenarios and selection comparison list the scenario "surge protective device status monitoring (retrofit of existing SPDs)," with a recommended combination of the FS surge protective device monitor, the ESM intelligent lightning-protection monitoring terminal (SPD monitor, e.g. ESM-11312-R), and the FSP SPD lightning-protection base (e.g. FSP-21000-R). This combination targets lightning-protection systems that are already built and in service—that is, adding status-monitoring capability without changing the original protection structure.
If the monitoring should fall directly on the protective device itself, the FSS intelligent surge protective device (e.g. FSS-11100) offers another choice. The leakage-current version of the FSS intelligent surge protective device (+leakage, OLED) provides leakage-current monitoring, in which the 2P type has 1 channel and the 4P type has 3 channels. That is, leakage-current monitoring can be built into the protective device itself rather than relying entirely on an add-on monitoring device.
If the protective devices of an existing system are already fixed and inconvenient to replace, add-on monitoring devices such as the FS surge protective device monitor, together with the FSP SPD lightning-protection base, are closer to the "retrofit" requirement; if multiple status quantities are to be acquired uniformly in terminal form, the ESM intelligent lightning-protection monitoring terminal takes the all-element monitoring position. There is no ranking among the three paths; the difference lies in the retrofitted object, the retrofit scope, and the data granularity.
4. How O&M actions should change
The real challenge posed by hidden degradation is not "whether to install monitoring," but that O&M actions must shift from "replace on failure" to "replace by condition." The traditional approach triggers action on whether equipment has failed or tripped. That is effective for functional links, but it has a blind spot for protection links: when a protective device degrades to a certain degree, the equipment still shows no abnormality, and the problem is exposed only when the next surge arrives.
With status quantities such as leakage current, temperature, and lifetime estimation, O&M judgment can shift from a single reading to trend comparison. The same leakage-current reading must be compared across different points in time before one can see whether it is stable, rising slowly, or already approaching a level that warrants attention. The same holds for temperature and lifetime estimation: a single value provides a snapshot, while continuous readings provide a trend. For lightning-protection project leads and facility O&M managers, the real point of reading the status values once after a thunderstorm is to add a critical data point to that trend line—because a thunderstorm is often a period when degradation advances relatively fast.
It should be emphasized that status quantities provide information related to protective capability; they cannot replace the periodic inspection and testing required by standards, nor do they constitute a commitment to the remaining life of a specific device. Their role is to turn "looks normal" into "verifiable with evidence."
Scope and limitations
- This article applies to status monitoring and O&M awareness of surge protective devices and related lightning-protection and grounding links in low-voltage distribution systems; it does not cover medium- or high-voltage systems or the specialized design of a specific project. - The product parameters cited (leakage current 50.0–1200.0 μA, accuracy ±10 μA, lifetime estimation 0–100%, etc.) apply only to the corresponding product models; the configuration of a specific model is subject to the actual order and accompanying documentation. - The leakage-current channel count of the FSS intelligent surge protective device (1 channel for the 2P type, 3 channels for the 4P type) applies only to the leakage-current version. - The recommended combination for surge protective device status monitoring (retrofit of existing SPDs)—FS surge protective device monitor / ESM intelligent lightning-protection monitoring terminal / FSP SPD lightning-protection base—is a reference for selection direction; it does not replace on-site survey and engineering calculation. - The relevant requirements for grounding links are based on GB 50057; this article is only an explanation from the perspective of status monitoring and O&M and does not constitute a compliance determination or acceptance conclusion. - This article contains no engineering parameters, certification information, or project cases beyond the documented product data; where any on-site data conflicts with this article, actual measurements and current standards shall prevail.
