As the UAW strikes keep disrupting automobile production at several Detroit Three plants in the US, it's business as usual for Tesla at its non-unionized Fremont Factory and Gigafactory Texas.

But the same cannot be said about Tesla's manufacturing operations at the Giga Berlin plant in Germany that builds the Model Y. A rising number of Tesla workers at the site are reportedly joining the IG Metall union over working conditions, the labor organization claims.

IG Metall said in a statement seen by Reuters and Bloomberg that Tesla employees are complaining about safety hazards, including extreme workloads caused by staff shortages and overly ambitious production targets.

The lack of staff and inadequate safety measures are said to lead to a high number of accidents at work, and situations where around 30 percent of workers were signed off sick are not rare, the union said.

IG Metall said more than 1,000 workers showed up at the plant on Monday wearing stickers calling for "safe and fair work." While the union does not typically reveal specific membership numbers for companies, it noted that it has seen a steep rise in the number of new members at Tesla's Giga Berlin. The site employs approximately 12,000 people. 

Gallery: Tesla Giga Berlin (Tesla Gigafactory 4)

Tesla managers reportedly invited their teams to a meeting with "free food and a surprise" on Sunday night to discuss IG Metall's presence on the site. "We want to speak with you and your teams about the questionable methods and actual goals of IG Metall," read the email seen by Reuters.

In response to Tesla's email to Giga Berlin employees, local IG Metall official Dirk Schulze said "no one in Germany needs to hide their union membership" as the right to organize in a union and and to openly stand up for it in the workplace is guaranteed by the German Constitution.

Twelve workers were approached by Reuters on Monday, and eight of them said pressure was too high, with some reporting high incidence of accidents and issues with receiving overtime pay. Four workers said they were satisfied with working conditions, while two said they were not allowed to speak to the media.

It will be interesting to see how Tesla deals with this reported increase in unionization activity at Giga Berlin knowing CEO Elon Musk's little tolerance for unions. Earlier this year, the EV maker terminated a group of Tesla workers in Buffalo, New York, a day after they launched a unionization campaign.

Tesla has so far refused to sign the kind of collective wage agreements that are standard in Germany, attracting strong criticism from IG Metall, which represents 2.2 million workers in the German metal and electrical industries.

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