I’m wondering if there is any downside to installing a whole home surge protector at the mains.
No...
What manufacture is the electrician going to install?
If the SPD (Surge Protection Device) will be mounted on the outside of the panel enclosure it is important to install it close to the main breaker . Install the 2 pole breaker for the SPD connection to the panel Hot bus in the two spaces just below the main breaker. Have the electrician relocate breaker(s) to free up the two spaces if needed for the SPD 2 pole breaker. Leads from the SPD to breaker must be kept short as possible. Also keep the SPD neutral lead as short as possible. Same for the SPD ground lead. Usually in a main electrical service panel the neutral bar is also used for the equipment grounding bar. Electrical service entrance neutral conductor is bonded to the metal enclosure as well the neutral is connected to mother earth.
Make sure the electrician inspects the electrical service’s Grounding Electrode System. He will know what that is. Have him inspect all connections. Clean and tighten as needed. Add a ground rod(s) if needed to the existing ground rod(s). The lower the electrode to earth resistance the better. NEC electrical safety code only requires 25 ohms or less. That is way too high for good lightning SPD protection. IEEE recommends 5 ohms or less...
Here is an example of a very good SPD. Siemens also makes good ones.
CHSPT2ULTRA | Eaton CHSPT2 surge protection device
Siemens "First Surge" SPD.
FirstSurge™ - Siemens Download Center
Here is a good video for you to watch. It shows why it is important to keep the SPD as close as possible to the bus of the electrical panel where the electrical power enters the panel and connects to the buss.
Surge Protection - Power Quality - Experience centers showcase
Also worth noting read the manufacturer’s warranty. As a rule the SPD must be purchased from an authorized dealer. Also the SPD must be installed by a licensed electrician.
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